+50 Microbiology USMLE Step 1 and NBME CBSE Style Questions and Answers with Explanations

1. A 67-year-old man is recovering from an influenza infection he had two weeks ago. He now presents with high fever, productive cough yielding blood-tinged sputum, and shortness of breath. A chest X-ray reveals a lobar consolidation in the right lung. Gram stain of the sputum shows gram-positive cocci in clusters. Which of the following organisms is the most likely cause of this patient's pneumonia?

A. Streptococcus pneumoniae
B. Haemophilus influenzae
C. Mycoplasma pneumoniae
D. Staphylococcus aureus
E. Klebsiella pneumoniae

Answer and Explanation: The correct answer is D. Staphylococcus aureus. Staphylococcus aureus is a well-known cause of post-influenza bacterial pneumonia​emcrit.org. Influenza virus can damage the respiratory epithelium and impair local defenses, which creates an opportunity for S. aureus to infect the lungs as a secondary invader. S. aureus is a gram-positive coccus in clusters that is coagulase-positive and often causes a rapidly progressive, necrotizing pneumonia with hemoptysis after viral illnesses. In contrast, Streptococcus pneumoniae (choice A) is the most common cause of community-acquired pneumonia overall but is not specifically associated with recent influenza. Haemophilus influenzae (choice B) typically causes pneumonia in smokers or those with COPD and is known for epiglottitis in unvaccinated children, not a classic post-viral pneumonia. Mycoplasma pneumoniae (choice C) causes atypical "walking" pneumonia in younger patients and is unlikely to cause a lobar pneumonia in an older adult post-influenza. Klebsiella pneumoniae (choice E) is associated with alcoholics and aspiration, often producing currant-jelly sputum, but it is not specifically linked to post-influenza infections.

2. An 18-year-old college student presents with a 1-week history of fever, sore throat, and fatigue. On examination, he has enlarged, tender posterior cervical lymph nodes and mild splenomegaly. A peripheral blood smear shows atypical lymphocytes (activated T cells), and a Monospot test is positive for heterophile antibodies. Which of the following is the most likely causative agent of this patient's condition?

A. Epstein-Barr virus (EBV)
B. Cytomegalovirus (CMV)
C. Acute HIV infection
D. Streptococcus pyogenes
E. Toxoplasma gondii

Answer and Explanation: The correct answer is A. Epstein-Barr virus (EBV). EBV is the causative agent of infectious mononucleosis, characterized by fever, pharyngitis, lymphadenopathy (especially posterior cervical), and splenomegaly. The Monospot test detects heterophile antibodies, which are typically positive in EBV infection​. EBV infects B cells via the CD21 receptor and triggers a robust cytotoxic T-cell response (atypical lymphocytes on smear) that causes the symptoms. Cytomegalovirus (choice B) can cause a mononucleosis-like syndrome but usually produces a heterophile antibody-negative Monospot test. Acute HIV infection (choice C) can also cause fever and lymphadenopathy but is usually accompanied by other signs (rash, mucosal ulcers) and does not cause heterophile-positive mononucleosis. Streptococcus pyogenes (choice D) causes streptococcal pharyngitis, which can present with sore throat and lymphadenopathy, but it does not cause atypical lymphocytes or a positive heterophile test. Toxoplasma gondii (choice E) infection can mimic mononucleosis in symptoms (especially in immunocompetent hosts) but would be heterophile-negative and is often acquired from undercooked meat or cat feces exposure.

3. A 10-year-old boy is brought to the clinic with a 2-week history of migratory joint pains and a rash. He recently recovered from a sore throat that was not treated with antibiotics. On exam, he has a low-grade fever, a new holosystolic murmur best heard at the apex (consistent with mitral regurgitation), and a ring-shaped erythematous rash on his trunk. These findings suggest rheumatic fever. Which of the following is the most likely mechanism responsible for his current condition?

A. Antibodies against streptococcal antigens cross-reacting with heart tissue
B. Deposition of immune complexes in cardiac endothelium
C. Superantigen activation of T cells releasing cytokines
D. Direct invasion of cardiac muscle by Streptococcus pyogenes
E. Delayed-type (Type IV) hypersensitivity causing granuloma formation

Answer and Explanation: The correct answer is A. Antibodies against streptococcal antigens cross-react with cardiac tissue, a phenomenon known as molecular mimicry. In rheumatic fever, antibodies generated against the Streptococcus pyogenes M protein during a strep throat infection mistakenly target cardiac myosin and other proteins in the heart, leading to inflammation of the endocardium, myocardium, and pericardium​amboss.com. This immune-mediated damage (a type II hypersensitivity reaction) explains the pancarditis and valve damage (classically affecting the mitral valve) seen in rheumatic fever. Immune complex deposition (choice B) is characteristic of post-streptococcal glomerulonephritis (type III hypersensitivity), not rheumatic fever. Superantigen activation (choice C) describes how certain staphylococcal or streptococcal toxins (like toxic shock syndrome toxin) cause massive cytokine release; it is not the mechanism of rheumatic fever. Direct bacterial invasion (choice D) is incorrect because rheumatic fever involves autoimmune damage after the infection, and there are no bacteria in affected tissues. Delayed-type hypersensitivity (choice E) involving granulomas (type IV hypersensitivity) is more characteristic of chronic infections like tuberculosis, not acute rheumatic fever (though Aschoff bodies in rheumatic fever are localized collections of inflammatory cells, the primary cause is still antibody cross-reactivity).

4. A 36-year-old man with AIDS (CD4 count 40/µL) presents with a 2-week history of headache, fever, and confusion. He has neck stiffness on exam. A lumbar puncture is performed. Cerebrospinal fluid (CSF) analysis shows low glucose, high protein, and lymphocytic pleocytosis. India ink staining of the CSF reveals round yeast cells with a thick capsule. Which of the following is the most likely cause of this patient's condition?

A. Neisseria meningitidis
B. Mycobacterium tuberculosis
C. Cryptococcus neoformans
D. Toxoplasma gondii
E. Coccidioides immitis

Answer and Explanation: The correct answer is C. Cryptococcus neoformans. Cryptococcus neoformans is an encapsulated yeast and the most common cause of fungal meningitis in AIDS patients. It is typically found in soil contaminated with pigeon droppings and is inhaled into the lungs before disseminating to the brain​. In the CSF, it appears as round yeasts with a characteristic thick polysaccharide capsule, which can be visualized as a clear halo on India ink stain. Diagnosis is often confirmed with a cryptococcal antigen test on CSF. Neisseria meningitidis (choice A) causes acute bacterial meningitis, usually in healthy young individuals living in close quarters; it is a gram-negative diplococcus and would show neutrophils in CSF, not a single budding yeast. Mycobacterium tuberculosis (choice B) can cause chronic meningitis in AIDS patients, but it is an acid-fast bacillus and would not appear as budding yeasts with capsules. Toxoplasma gondii (choice D) causes brain abscesses (ring-enhancing lesions) in AIDS patients, not meningitis with yeast in CSF. Coccidioides immitis (choice E) is a dimorphic fungus that can cause meningitis, especially in the Southwest US, but in tissue it forms spherules with endospores (not yeast with a capsule) and is less common than Cryptococcus in AIDS patients.

5. A 22-year-old sexually active woman presents with dysuria, urinary frequency, and urgency for the past 2 days. She recently returned from her honeymoon. She has no fever or flank pain. Urinalysis shows numerous white blood cells and is positive for nitrites. Which of the following is the most likely causative organism?

A. Staphylococcus saprophyticus
B. Escherichia coli
C. Proteus mirabilis
D. Enterococcus faecalis
E. Klebsiella pneumoniae

Answer and Explanation: The correct answer is B. Escherichia coli. E. coli is the most common cause of uncomplicated urinary tract infections in young women​. It is a gram-negative rod from the gastrointestinal tract that can colonize the periurethral area and ascend into the bladder. The positive nitrite test in the urine is also suggestive of a gram-negative organism that reduces nitrates (as E. coli does). Staphylococcus saprophyticus (choice A) is the second most common cause of UTIs in sexually active young women, but it is a gram-positive organism and typically nitrite-negative. Proteus mirabilis (choice C) can cause UTIs, often associated with struvite kidney stones and a characteristic ammonia scent in the urine, but E. coli is far more common in this scenario. Enterococcus faecalis (choice D) is an uncommon cause of UTI in healthy young women (more often seen in older hospitalized patients or after instrumentation). Klebsiella pneumoniae (choice E) can cause UTIs, especially in hospital settings, but it is not as frequent a cause of uncomplicated cystitis as E. coli.

6. A 58-year-old man steps on a rusty nail while working on a farm. He did not seek medical care at the time and is unsure of his last tetanus booster. Two weeks later, he develops painful muscle spasms in his jaw (lockjaw) and back (opisthotonus), and has trouble swallowing. Which of the following best describes the mechanism of the toxin responsible for this patient's symptoms?

A. Inhibition of acetylcholine release at the neuromuscular junction
B. Activation of adenylate cyclase leading to increased cAMP in enterocytes
C. Blockade of inhibitory neurotransmitter release in the central nervous system
D. Inactivation of elongation factor-2, halting host cell protein synthesis
E. Superantigen-mediated widespread T lymphocyte activation

Answer and Explanation: The correct answer is C. Blockade of inhibitory neurotransmitter release in the CNS. Tetanus toxin (tetanospasmin), produced by Clostridium tetani, is a protease that cleaves synaptobrevin, preventing the release of inhibitory neurotransmitters glycine and GABA from Renshaw cells in the spinal cord. The loss of inhibition leads to the uncontrolled muscle spasms and spastic paralysis seen in tetanus. Inhibition of acetylcholine release (choice A) is the mechanism of botulinum toxin, which causes flaccid paralysis, not spastic paralysis. Activation of adenylate cyclase (choice B) leading to increased cAMP is the mechanism of toxins like cholera toxin and the heat-labile toxin of E. coli, which cause watery diarrhea. Inactivation of elongation factor-2 (choice D) is the mechanism of diphtheria toxin (and Pseudomonas exotoxin A), leading to cell death, not the cause of tetanic muscle contractions. Superantigen-mediated T-cell activation (choice E) describes toxins like toxic shock syndrome toxin (TSST-1) from S. aureus or erythrogenic toxin A from Streptococcus pyogenes, which cause toxic shock syndrome or scarlet fever, not tetanus.

7. A 34-year-old woman is treated with clindamycin for an anaerobic infection. One week into therapy, she develops abdominal cramping and profuse watery diarrhea. Colonoscopy reveals yellow-white plaques on the colonic mucosa. Stool studies detect toxins in the stool. Which of the following organisms is most likely responsible for this patient's diarrhea?

A. Giardia lamblia
B. Staphylococcus aureus
C. Clostridium perfringens
D. Clostridioides difficile
E. Campylobacter jejuni

Answer and Explanation: The correct answer is D. Clostridioides difficile (formerly Clostridium difficile). Antibiotic use (especially clindamycin, fluoroquinolones, or broad-spectrum antibiotics) can disrupt normal gut flora and allow overgrowth of C. difficile​mayoclinic.org. C. difficile produces exotoxins (toxin A and toxin B) that damage the colonic mucosa and lead to pseudomembranous colitis, characterized by diarrhea and yellow-white plaques on the colon. Giardia lamblia (choice A) causes greasy, foul-smelling diarrhea from drinking contaminated water (e.g., streams) and is not related to antibiotic use. Staphylococcus aureus (choice B) can cause food poisoning with vomiting and diarrhea via pre-formed toxins in improperly stored foods, but it does not cause pseudomembranous colitis. Clostridium perfringens (choice C) can cause gas gangrene and also food poisoning (late-onset diarrhea from reheated meats), but it is not associated with antibiotic-related colitis. Campylobacter jejuni (choice E) is a common cause of inflammatory diarrhea (often bloody) from undercooked poultry, not typically associated with prior antibiotic use or pseudomembrane formation.

8. A 32-year-old man returns from a trip to rural Nigeria with a 1-week history of high fevers, chills, and sweats. He notes that the fevers tend to spike periodically. On exam, he is febrile and pale, and his spleen is enlarged. Lab tests show hemolytic anemia. His urine appears dark. A peripheral blood smear demonstrates multiple ring-form parasites within red blood cells. Which of the following is the most likely cause of his symptoms?

A. Plasmodium falciparum
B. Babesia microti
C. Trypanosoma brucei
D. Leishmania donovani
E. Dengue virus

Answer and Explanation: The correct answer is A. Plasmodium falciparum. P. falciparum is the most deadly of the malaria parasites and is prevalent in sub-Saharan Africa. It infects red blood cells of all ages, often leading to high levels of parasitemia; multiple ring-form trophozoites can be seen inside a single RBC on blood smear​path.upmc.edu. P. falciparum can cause an irregular fever pattern (often daily or cyclic fevers without a strict 48-hour periodicity) and severe complications like cerebral malaria (confusion, seizures) and hemoglobinuria (blackwater fever). Babesia microti (choice B) is a tick-borne parasite (endemic to the northeastern US) that can cause malaria-like illness with fever and hemolysis, but it is geographically unlikely given this patient’s travel to Nigeria and typically shows a "Maltese cross" formation in RBCs on smear. Trypanosoma brucei (choice C) causes African sleeping sickness (transmitted by the tsetse fly) characterized by recurrent fevers, lymphadenopathy, and progressive confusion and somnolence, rather than the cyclic fevers with hemolytic anemia seen here. Leishmania donovani (choice D) causes visceral leishmaniasis (kala-azar), leading to fever, weight loss, hepatosplenomegaly, and pancytopenia; it does not appear as ring forms inside RBCs. Dengue virus (choice E) causes dengue fever ("breakbone fever") with high fevers, severe muscle/joint pain, and sometimes hemorrhagic manifestations, but it is a viral illness diagnosed by serology and does not show intraerythrocytic parasites on blood smear.

9. A 45-year-old homeless man presents with chronic cough productive of bloody sputum, unintentional weight loss, night sweats, and fatigue over the last 3 months. He has a 20-pack-year smoking history and occasional alcohol use. Physical exam reveals apical crackles in the lungs. A chest X-ray shows cavitary lesions in the upper lobes. Sputum smears with special staining show pink-red rods. Which of the following is the most likely causative organism?

A. Actinomyces israelii
B. Nocardia asteroides
C. Histoplasma capsulatum
D. Mycobacterium tuberculosis
E. Blastomyces dermatitidis

Answer and Explanation: The correct answer is D. Mycobacterium tuberculosis. The patient's clinical picture of chronic cough with hemoptysis, night sweats, weight loss, and apical cavitary lesions is classic for pulmonary tuberculosis. M. tuberculosis is an acid-fast bacillus (rod) with a waxy cell wall rich in mycolic acids, which makes it stain red/pink with the Ziehl-Neelsen (acid-fast) stain. It spreads via respiratory droplets and commonly reactivates in the lung apices of immunocompromised or malnourished individuals (such as the homeless). Actinomyces israelii (choice A) causes chronic cervicofacial ("lumpy jaw") infections with sinus tracts draining sulfur granules; it does not typically infect the lungs except by aspiration (and even then causes abscesses, not cavitary TB-like lesions). Nocardia asteroides (choice B) is a filamentous branching rod that is weakly acid-fast and can cause pulmonary infections that mimic TB, but it usually affects immunocompromised patients (e.g., on steroids) and can disseminate to the brain; our patient’s scenario and risk factors are more consistent with TB. Histoplasma capsulatum (choice C) is a dimorphic fungus found in the Ohio and Mississippi River valleys, causing granulomatous lung disease that can mimic TB, but it is associated with bird or bat droppings exposure and the yeast forms are found within macrophages (also, the stain mentioned in the question indicates bacteria, not fungi). Blastomyces dermatitidis (choice E) is another dimorphic fungus (in the Mississippi/Ohio river valleys and Great Lakes region) that causes lung infection and can form granulomas; it is characterized by broad-based budding yeasts in tissue. However, the epidemiology and classic acid-fast stain findings here point to M. tuberculosis as the cause.

10. A 25-year-old man presents with a 1-week history of jaundice, fever, nausea, and abdominal pain. Three weeks ago, he returned from a backpacking trip through rural Mexico, during which he frequently ate food from street vendors. He has no significant medical history and does not use IV drugs. On exam, he has yellowing of the sclerae and mild right upper quadrant tenderness. Lab tests show elevated liver enzymes. Which of the following is the most likely cause of his symptoms?

A. Hepatitis B virus
B. Hepatitis C virus
C. Hepatitis A virus
D. Hepatitis D virus
E. Hepatitis E virus

Answer and Explanation: The correct answer is C. Hepatitis A virus (HAV). Hepatitis A is a picornavirus (ssRNA, positive-sense, non-enveloped) transmitted via the fecal-oral route, often through contaminated food or water in regions with poor sanitation. This patient’s travel history and acute onset of jaundice and hepatitis symptoms are classic for HAV infection. HAV typically causes a self-limited acute hepatitis that does not progress to chronic liver disease​mayoclinic.org. Hepatitis B virus (choice A) is transmitted via blood or body fluids (e.g., unprotected sex, IV drug use) and can cause both acute and chronic hepatitis, but it is less likely in a traveler who consumed local food. Hepatitis C virus (choice B) is primarily blood-borne (IV drug use, transfusions) and usually causes chronic hepatitis rather than an acute, symptomatic illness in a young traveler. Hepatitis D virus (choice D) is a defective virus that depends on co-infection with HBV; it would not cause illness on its own in this scenario. Hepatitis E virus (choice E) is another fecal-oral transmitted virus that can cause acute hepatitis, often in areas like Asia or Mexico; it is similar to HAV but is particularly severe in pregnant women. In this patient, HAV is the most likely cause of acute hepatitis after travel.

11. A hospitalized 30-year-old man with severe burns develops an infection of his burn wounds. The affected skin has a blue-green purulent discharge and a fruity, grape-like odor. Which of the following organisms is the most likely cause of this patient's wound infection?

A. Streptococcus pyogenes
B. Staphylococcus aureus
C. Clostridium perfringens
D. Pseudomonas aeruginosa
E. Escherichia coli

Answer and Explanation: The correct answer is D. Pseudomonas aeruginosa. Pseudomonas is a common cause of wound and skin infections in burn patients​. It is a motile, aerobic, gram-negative rod that is non-lactose fermenting and oxidase positive. It produces characteristic pigments (pyocyanin and pyoverdin) that can give infected tissue a blue-green color and emits a fruity, grape-like odor. Streptococcus pyogenes (choice A) can cause severe skin infections like necrotizing fasciitis, but it does not typically produce a colored pigment or fruity odor. Staphylococcus aureus (choice B) is another common cause of wound infections, including in burns, but it typically produces golden-yellow colonies and no specific fruity smell. Clostridium perfringens (choice C) causes gas gangrene in deep wounds, leading to crepitus (gas in tissue) and necrosis, not a green discharge in burns. Escherichia coli (choice E) is a gram-negative rod that mainly causes UTIs and intra-abdominal infections; it is not a typical pathogen in burn wound infections.

12. A 35-year-old man living in Arizona presents with 3 weeks of cough, fever, and joint aches. He also noticed tender red bumps on his shins. On exam, he has erythema nodosum on the lower legs. Chest X-ray shows a left upper lobe infiltrate. A biopsy of a lung nodule reveals thick-walled spherules filled with endospores. Which of the following is the most likely causative organism?

A. Histoplasma capsulatum
B. Blastomyces dermatitidis
C. Mycobacterium tuberculosis
D. Streptococcus pneumoniae
E. Coccidioides immitis

Answer and Explanation: The correct answer is E. Coccidioides immitis. Coccidioides is a dimorphic fungus endemic to the desert Southwest (e.g., Arizona, California) and is the cause of "Valley fever." The patient’s combination of fever, arthralgias, and erythema nodosum is known as “desert rheumatism” and is characteristic of primary coccidioidal infection​ncbi.nlm.nih.gov. In the lungs, Coccidioides forms spherules packed with endospores (as seen on biopsy). Infection occurs from inhaling airborne arthroconidia (spores) in soil, often after dust exposure. Histoplasma capsulatum (choice A) is found in the Ohio and Mississippi River valleys and causes a TB-like lung infection; it is associated with bird or bat droppings and appears as small yeasts within macrophages, not spherules with endospores. Blastomyces dermatitidis (choice B) is found in states east of the Mississippi and the Great Lakes region; it causes pulmonary infection with broad-based budding yeasts and can cause skin lesions. Mycobacterium tuberculosis (choice C) can cause fever, cough, and nodular lung lesions, but it is not associated with erythema nodosum in the desert Southwest, and biopsy would show caseating granulomas with acid-fast bacilli, not fungal spherules. Streptococcus pneumoniae (choice D) is the most common cause of community-acquired pneumonia but typically causes an acute lobar pneumonia and would not explain the erythema nodosum or spherules on biopsy.

13. A baby is born with hydrocephalus, chorioretinitis, and intracranial calcifications. The mother recalls having a flulike illness during pregnancy and was noted to have lymphadenopathy. She has several pet cats at home and admits she did not avoid cleaning the litter box while pregnant. Which of the following infectious agents is the most likely cause of the baby's condition?

A. Cytomegalovirus
B. Rubella virus
C. Listeria monocytogenes
D. Treponema pallidum
E. Toxoplasma gondii

Answer and Explanation: The correct answer is E. Toxoplasma gondii. Congenital toxoplasmosis classically presents with a triad of chorioretinitis, hydrocephalus, and intracranial calcifications​radiopaedia.org. T. gondii is a protozoan parasite that can infect a pregnant woman (often via exposure to oocysts in cat feces or ingestion of undercooked meat) and then cross the placenta to the fetus. Maternal infection is usually mild or asymptomatic, but fetal infection can lead to severe neurologic and ocular damage. Cytomegalovirus (choice A) is the most common overall TORCH infection and can cause congenital hearing loss, seizures, and periventricular calcifications (not diffuse intracranial calcifications), but the triad described here is more specific for toxoplasmosis. Rubella virus (choice B) causes congenital rubella syndrome (cataracts, deafness, cardiac defects like PDA, and "blueberry muffin" rash) and is prevented by maternal vaccination; it does not cause hydrocephalus or intracranial calcifications. Listeria monocytogenes (choice C) can cause neonatal sepsis and meningitis (often from contaminated food in pregnancy), but it does not cause the triad of findings described. Treponema pallidum (choice D), which causes congenital syphilis, leads to features like snuffles (rhinitis), saber shins, Hutchinson teeth, and rash; it does not present with hydrocephalus or intracranial calcifications.

14. A 6-year-old boy is evaluated for facial puffiness and tea-colored urine. Two weeks ago, he had an impetigo infection with honey-crusted lesions around his mouth, which was caused by group A Streptococcus. He was not treated with antibiotics. His blood pressure is now elevated, and urinalysis shows hematuria and red blood cell casts. Which of the following is the most likely mechanism for his current condition?

A. Autoimmune cross-reactivity between bacterial and glomerular antigens
B. Bacterial invasion and colonization of glomerular tissue
C. Deposition of immune complexes in the glomeruli with complement activation
D. IgE-mediated mast cell degranulation in renal microvasculature
E. Preformed bacterial toxin acting on the kidney

Answer and Explanation: The correct answer is C. Deposition of immune complexes in the glomeruli with complement activation. Acute poststreptococcal glomerulonephritis is a classic example of a type III hypersensitivity reaction​osmosis.org. After a streptococcal skin or throat infection, immune complexes (formed by antibodies bound to streptococcal antigens) deposit in the glomerular basement membrane, leading to complement activation and inflammation. Clinically this results in nephritic syndrome: hematuria (tea- or cola-colored urine), hypertension, edema, and renal impairment. Autoimmune cross-reactivity (choice A) describes molecular mimicry as seen in rheumatic fever (type II hypersensitivity) affecting the heart, not the kidney in this case. Bacterial invasion of glomeruli (choice B) is not how PSGN occurs; the glomeruli are damaged by the immune response, not direct infection. IgE-mediated mast cell degranulation (choice D) is characteristic of type I hypersensitivity (allergies) and has no role in PSGN. Preformed bacterial toxin (choice E) is not the cause of glomerular damage here; rather, it’s the immune complexes and resulting inflammation that cause the injury.

15. A 30-year-old man was exploring caves in the southwestern United States when he was bitten by a bat. He cleans the wound and goes to the emergency department immediately. The bat is not captured. The patient has no symptoms and has never received rabies vaccination. Which of the following is the most appropriate next step in management to prevent rabies infection?

A. Administer both passive immunization (human rabies immunoglobulin) and active vaccination
B. Start oral acyclovir therapy immediately
C. Observe the patient and initiate treatment only if symptoms develop
D. Give a tetanus toxoid booster and no further action
E. Administer broad-spectrum antibiotics to prevent secondary infection

Answer and Explanation: The correct answer is A. Administer both passive immunization (HRIG) and active vaccination. Bat bites are high risk for rabies exposure, and post-exposure prophylaxis is indicated even if the patient has no symptoms​. Rabies post-exposure prophylaxis for an unvaccinated individual consists of thorough wound cleansing, injection of human rabies immune globulin (around the wound and intramuscularly), and a series of rabies vaccinations (active immunization). This combined approach is extremely effective at preventing the virus from establishing infection if given promptly. Acyclovir (choice B) is not effective against rabies (it is used for herpesvirus infections). Observation without treatment (choice C) would be inappropriate because once rabies symptoms begin, the disease is almost invariably fatal, so prophylaxis must be given before symptom onset. Tetanus toxoid (choice D) may be given if the patient’s tetanus immunization is not up to date, but it does not prevent rabies. Broad-spectrum antibiotics (choice E) are not useful for preventing rabies, which is caused by a virus; wound cleansing and rabies-specific prophylaxis are the priorities.

16. A region is experiencing an outbreak of poliomyelitis, and health officials are considering vaccination strategies to not only protect individuals from paralysis but also to halt fecal-oral transmission of the poliovirus. Which of the following vaccine approaches would provide the greatest mucosal (secretory IgA) immunity in the intestines?

A. Killed (inactivated) poliovirus vaccine given by intramuscular injection
B. Toxoid vaccine targeting the poliovirus neurotoxin
C. Live attenuated poliovirus vaccine given orally
D. Passive immunization with pooled anti-poliovirus antibodies
E. Live attenuated intranasal poliovirus vaccine

Answer and Explanation: The correct answer is C. Live attenuated poliovirus vaccine given orally. The oral polio vaccine (OPV, Sabin vaccine) is a live attenuated virus that replicates in the gut and induces a strong local immune response, including secretory IgA in the intestinal mucosa​. This mucosal immunity helps prevent poliovirus from infecting the GI tract and reduces viral shedding, thereby interrupting transmission in a community. In contrast, the inactivated polio vaccine (IPV) given by injection (choice A) induces good serum IgG immunity to prevent paralytic disease but generates little IgA in the gut, so the virus can still infect and be shed from the intestines. A toxoid vaccine (choice B) is not applicable to polio, since polio pathology is due to viral infection of neurons, not a secreted toxin. Passive antibody administration (choice D) could temporarily provide immunity but is not a practical or long-lasting strategy for community-wide protection. An intranasal polio vaccine (choice E) does not exist; however, there is a live attenuated intranasal vaccine for influenza (not polio). The OPV (oral live vaccine) is uniquely effective in inducing intestinal IgA immunity.

17. A 19-year-old college student living in a dormitory presents with sudden onset of high fever, headache, neck stiffness, and a purplish spotted rash on his trunk and extremities. He appears confused. His blood pressure is 80/50 mmHg and pulse is 120/min. Lumbar puncture reveals cloudy cerebrospinal fluid with many neutrophils and gram-negative diplococci. Which of the following is the most likely causative organism?

A. Streptococcus pneumoniae
B. Haemophilus influenzae type b
C. Listeria monocytogenes
D. Escherichia coli
E. Neisseria meningitidis

Answer and Explanation: The correct answer is E. Neisseria meningitidis. This presentation – acute bacterial meningitis in a young adult with a petechial/purpuric rash and signs of septic shock – is classic for meningococcal meningitis. N. meningitidis is a gram-negative diplococcus that colonizes the nasopharynx and can spread in close-contact settings like dormitories or military barracks. It has a polysaccharide capsule (the basis for vaccines) and its endotoxin (lipooligosaccharide) can cause disseminated intravascular coagulation and shock (Waterhouse-Friderichsen syndrome). Streptococcus pneumoniae (choice A) is a common cause of meningitis, especially in older adults, but typically does not cause a rash. Haemophilus influenzae type b (choice B) used to be a common pediatric meningitis cause before the Hib vaccine; it’s less likely in a 19-year-old, and also typically not associated with a purpuric rash. Listeria monocytogenes (choice C) causes meningitis in neonates, the elderly, or immunocompromised patients (often via contaminated food) rather than healthy young adults. Escherichia coli (choice D) is a leading cause of neonatal meningitis (especially those with the K1 capsular antigen) but is not a typical cause in teenagers or adults.

18. A 25-year-old woman is brought to the emergency department with high fever, vomiting, and a diffuse red rash. Her blood pressure is 70/40 mmHg. On exam, the rash resembles a sunburn and involves her palms and soles. She is noted to be using super-absorbent tampons. Toxic shock syndrome is suspected. Which of the following best describes the mechanism by which the responsible toxin causes this condition?

A. Nonspecific activation of T lymphocytes leading to massive cytokine release
B. Release of endotoxin triggering macrophage Toll-like receptor 4
C. ADP-ribosylation of host elongation factor-2 in T cells
D. Inactivation of inhibitory interneurons in the spinal cord
E. Deposition of antigen–antibody complexes in peripheral capillaries

Answer and Explanation: The correct answer is A. Nonspecific activation of T lymphocytes leading to massive cytokine release. Staphylococcus aureus toxic shock syndrome toxin-1 (TSST-1) is a superantigen that simultaneously binds to MHC II on antigen-presenting cells and the Vβ region of T-cell receptors, activating a large number of T cells non-specifically​. This causes a burst of cytokines (IL-2 from T cells and IL-1, TNF-α from macrophages), leading to fever, rash, capillary leak, hypotension, and multi-organ effects characteristic of toxic shock syndrome. Endotoxin triggering TLR4 (choice B) describes the mechanism of septic shock caused by gram-negative bacteria (lipopolysaccharide on Gram-negative rods), not the superantigen-mediated shock in this scenario. ADP-ribosylation of EF-2 (choice C) is the mechanism of diphtheria toxin (and Pseudomonas exotoxin A), which halts protein synthesis, not a T cell activation mechanism. Inactivation of spinal inhibitory interneurons (choice D) describes tetanus toxin’s effect on neurotransmitter release, not TSST-1. Immune complex deposition (choice E) is associated with type III hypersensitivity (e.g., serum sickness, poststreptococcal glomerulonephritis) and is not the cause of toxic shock syndrome.

19. An otherwise healthy 28-year-old man has been repeatedly exposed to HIV through unprotected intercourse with an HIV-positive partner, yet he remains HIV-negative on multiple tests. His partner’s viral load is high. Genetic testing reveals a mutation that is likely responsible for this apparent resistance to HIV infection. Which of the following genetic abnormalities is the most likely explanation?

A. A loss-of-function mutation in the CD4 gene on helper T cells
B. A homozygous CCR5 chemokine receptor mutation (Δ32 deletion)
C. A mutation in the CXCR4 chemokine receptor gene
D. A mutation in the HIV reverse transcriptase gene
E. Overexpression of neutralizing antibodies against HIV

Answer and Explanation: The correct answer is B. A homozygous CCR5 chemokine receptor mutation (Δ32 deletion). Some individuals carry a 32-base pair deletion in the CCR5 gene, resulting in a nonfunctional CCR5 co-receptor on macrophages​. HIV (particularly the macrophage-tropic, or R5 strains) uses CCR5 as a co-receptor (along with CD4) to enter host cells, so homozygous CCR5-Δ32 individuals are highly resistant to HIV infection. CD4 mutations (choice A) are not known to confer resistance and would likely impair immune function significantly. CXCR4 (choice C) is another co-receptor used by T-tropic HIV strains (typically in later infection); a mutation there could affect later-stage infection, but CCR5 is crucial for initial infection by most strains. A reverse transcriptase mutation in HIV (choice D) could make the virus drug-resistant or less fit, but this is a viral mutation, not a host genetic trait. Overexpression of antibodies (choice E) is not a known mechanism of resistance to initial infection; while some people develop broadly neutralizing antibodies, this occurs after infection, not as a primary resistance mechanism.

20. A 20-year-old man has had two episodes of Neisseria meningitidis bloodstream infection and meningitis in the past year. He has otherwise been healthy, and his vaccinations are up to date. Which of the following components of the immune system is most likely deficient in this patient?

A. C1 esterase inhibitor
B. Terminal complement components (C5–C9)
C. IgA immunoglobulin
D. IL-12 receptor
E. NADPH oxidase enzyme

Answer and Explanation: The correct answer is B. Terminal complement components (C5–C9). Deficiency of the late complement components (which form the membrane attack complex) predisposes to recurrent infections with Neisseria species​. The membrane attack complex (MAC) is important for lysing gram-negative bacteria like Neisseria; without any one of C5, C6, C7, C8, or C9, a person can have frequent Neisserial infections despite otherwise intact immunity. C1 esterase inhibitor deficiency (choice A) causes hereditary angioedema, not specifically recurrent Neisseria infections. Selective IgA deficiency (choice C) can lead to recurrent sinopulmonary and GI infections (often with Giardia) due to lack of mucosal IgA, but it is not specifically associated with Neisseria infections. IL-12 receptor deficiency (choice D) impairs the Th1 immune response and IFN-γ production, leading to disseminated mycobacterial infections, not meningococcal infections. NADPH oxidase deficiency (choice E) is chronic granulomatous disease, which causes recurrent infections with catalase-positive organisms (e.g., Staphylococcus, Serratia) due to impaired respiratory burst in phagocytes, and is not specifically linked to Neisseria.

21. A 4-year-old boy develops abdominal cramps and bloody diarrhea two days after eating an undercooked hamburger at a family barbecue. He is brought to the hospital when his urine output decreases and he becomes pale and irritable. Lab tests show hemolytic anemia, thrombocytopenia, and acute kidney injury. Which of the following organisms is the most likely cause of this patient's illness?

A. Shigella dysenteriae
B. Campylobacter jejuni
C. Enterohemorrhagic Escherichia coli (EHEC) O157:H7
D. Salmonella enteritidis
E. Enterotoxigenic Escherichia coli (ETEC)

Answer and Explanation: The correct answer is C. Enterohemorrhagic Escherichia coli (EHEC) O157:H7. This strain of E. coli is classically associated with eating undercooked ground beef and causes a severe bloody diarrhea​. In some patients (especially young children), EHEC produces Shiga-like toxins that lead to hemolytic-uremic syndrome (HUS), characterized by hemolytic anemia, thrombocytopenia, and renal failure. Shigella dysenteriae (choice A) also produces a Shiga toxin and can cause dysentery and HUS, but infection is usually from person-to-person spread or contaminated water/food (not specifically undercooked beef), and Shigella typically causes fever as well. Campylobacter jejuni (choice B) is a common cause of bloody diarrhea (often from undercooked poultry) and can precede Guillain-Barré syndrome, but it is less commonly associated with HUS. Salmonella enteritidis (choice D) causes gastroenteritis from undercooked eggs or poultry, but typically causes a febrile diarrhea without HUS. Enterotoxigenic E. coli (ETEC) (choice E) causes traveler's diarrhea (watery diarrhea from contaminated water) via heat-labile and heat-stable toxins and does not cause bloody diarrhea or HUS.

22. A 40-year-old man from Egypt presents with blood in his urine and dysuria. He recalls having had "snail fever" (schistosomiasis) in his youth. Cystoscopy reveals inflammation and fibrosis of the bladder wall, and biopsy shows squamous cell carcinoma of the bladder. Which of the following parasites is most likely responsible for his condition?

A. Enterobius vermicularis
B. Schistosoma mansoni
C. Schistosoma haematobium
D. Schistosoma japonicum
E. Clonorchis sinensis

Answer and Explanation: The correct answer is C. Schistosoma haematobium. S. haematobium is a blood fluke (trematode) that targets the venous plexus of the bladder. Chronic infection leads to granulomatous inflammation, bladder wall fibrosis, and is strongly associated with squamous cell carcinoma of the bladder​. It is endemic in parts of Africa and the Middle East; exposure occurs through freshwater containing snails (the intermediate host). Enterobius vermicularis (choice A) is the pinworm causing perianal itching in children, unrelated to bladder disease. Schistosoma mansoni (choice B) and S. japonicum (choice D) typically reside in mesenteric veins and cause intestinal and hepatic schistosomiasis (e.g., portal hypertension from liver fibrosis), not urinary disease. Clonorchis sinensis (choice E) is a liver fluke associated with biliary tract inflammation and cholangiocarcinoma, not bladder cancer.

23. An 8-year-old boy is brought to the clinic with a high fever, cough, runny nose, and red eyes. His mother notes that he has not been vaccinated. On exam, he has conjunctivitis and small white spots with a bluish center on his buccal mucosa. A day later, he develops a maculopapular rash that starts on his face and spreads downward. Which of the following is the most likely cause of this illness?

A. Rubella virus
B. Parvovirus B19
C. Measles (rubeola) virus
D. Parainfluenza virus
E. Coxsackievirus A

Answer and Explanation: The correct answer is C. Measles (rubeola) virus. This boy’s presentation – the three C’s (cough, coryza, conjunctivitis), Koplik spots in the mouth, and a descending maculopapular rash – is classic for measles. Measles is caused by a paramyxovirus (rubeola virus) and typically affects unvaccinated children. Rubella virus (choice A) causes German measles, which also involves a rash but is generally milder (lower fever, shorter duration) and often accompanied by tender postauricular lymphadenopathy; rubella does not cause Koplik spots or the severe prodrome seen in measles. Parvovirus B19 (choice B) causes fifth disease (erythema infectiosum, “slapped-cheek” rash) in children and can cause arthralgias in adults, but it does not typically cause the diffuse rash with postauricular lymphadenopathy described here (those findings point to rubella, not parvovirus). Parainfluenza virus (choice D) is a paramyxovirus that causes croup (laryngotracheobronchitis) characterized by a barking cough and inspiratory stridor, not the generalized rash of measles. Coxsackievirus A (choice E) causes hand-foot-and-mouth disease, with oral ulcers and a rash on the hands and feet, but it is distinct from measles and lacks the features described here.

24. A 19-year-old college student presents with 2 weeks of dry cough, headache, and low-grade fever. He continues to attend classes, as his symptoms are mild ("walking pneumonia"). Chest X-ray shows patchy infiltrates that appear worse than his clinical presentation would suggest. Laboratory tests reveal elevated cold agglutinin titers (IgM antibodies that agglutinate red blood cells at low temperatures). Which of the following is the most likely causative organism?

A. Streptococcus pneumoniae
B. Mycoplasma pneumoniae
C. Chlamydophila pneumoniae
D. Legionella pneumophila
E. Adenovirus

Answer and Explanation: The correct answer is B. Mycoplasma pneumoniae. Mycoplasma pneumoniae is a common cause of atypical ("walking") pneumonia in young adults, often in settings like college dorms or military barracks. It typically causes a mild, prolonged respiratory illness with an X-ray that looks worse than the patient. M. pneumoniae lacks a cell wall and is known to cause the production of cold agglutinins (IgM antibodies that can agglutinate RBCs at cool temperatures)​. Streptococcus pneumoniae (choice A) causes typical lobar pneumonia with high fever, productive cough, and often more severe symptoms. Chlamydophila pneumoniae (choice C) can also cause atypical pneumonia in young adults, but cold agglutinins are classically associated with Mycoplasma. Legionella pneumophila (choice D) causes atypical pneumonia (often severe) in older adults or those with exposure to contaminated water systems and typically presents with high fever, GI symptoms, and hyponatremia. Adenovirus (choice E) can cause pneumonia, but it more commonly causes pharyngitis or conjunctivitis; it is not associated with cold agglutinins or walking pneumonia in this age group.

25. A 45-year-old woman comes to the emergency department with blurred vision, dry mouth, and difficulty swallowing. Over the next few hours, she develops symmetric descending muscle weakness, starting with the face and progressing to the upper limbs. She has no fever or sensory deficits. History reveals that she ate home-canned vegetables a day before symptom onset. Which of the following is the most likely cause of her condition?

A. Clostridium tetani infection from a puncture wound
B. Clostridium perfringens toxin from reheated meat
C. Campylobacter jejuni infection leading to Guillain-Barré syndrome
D. Clostridium botulinum toxin from improperly canned food
E. Listeria monocytogenes from contaminated dairy

Answer and Explanation: The correct answer is D. Clostridium botulinum toxin from improperly canned food. Foodborne botulism is caused by ingesting preformed botulinum toxin, often from improperly canned or preserved foods. Botulinum toxin causes a neuroparalytic illness characterized by the "4 D's": dysphagia, diplopia, dry mouth, and dysarthria, followed by a symmetric, descending flaccid paralysis​. It acts by blocking acetylcholine release at the neuromuscular junction. Clostridium tetani (choice A) causes tetanus (spastic paralysis), not the flaccid paralysis seen here. Clostridium perfringens (choice B) can cause food poisoning (late-onset diarrhea) from reheated meats, or gas gangrene in wounds, but it does not cause neuro symptoms. Campylobacter jejuni (choice C) can trigger Guillain-Barré syndrome, an autoimmune demyelinating polyneuropathy that causes ascending paralysis (starting in the legs), whereas botulism causes descending paralysis. Listeria monocytogenes (choice E) from dairy would be more likely to cause meningitis (especially in elderly or immunocompromised patients), not a flaccid paralysis.

26. A 36-year-old man with advanced HIV (CD4 count 50 cells/µL) presents with pain on swallowing and substernal burning. On examination, there are white plaques on his tongue and oral mucosa. Endoscopy reveals white, cottage cheese–like plaques in the esophagus that can be scraped off, with underlying mucosal erythema. Which of the following is the most likely cause of his esophagitis?

A. Cytomegalovirus (CMV)
B. Candida albicans
C. Herpes simplex virus (HSV)
D. Cryptosporidium parvum
E. Mycobacterium avium complex

Answer and Explanation: The correct answer is B. Candida albicans. Candida is the most common cause of esophagitis in AIDS patients​hopkinsguides.com and is characterized by white plaques that can be scraped off. In immunocompromised patients, Candida causes thrush in the oropharynx and can extend into the esophagus (esophageal candidiasis, an AIDS-defining illness). Cytomegalovirus (choice A) can also cause esophagitis in AIDS, but endoscopy typically shows large linear ulcers rather than white plaques. Herpes simplex virus (choice C) causes esophagitis with small, deep ulcers and often coalescing "volcano-like" lesions. Cryptosporidium (choice D) causes chronic diarrhea in AIDS, not esophagitis. Mycobacterium avium complex (choice E) causes disseminated infection (fevers, weight loss, anemia) in very low CD4 counts, not specifically esophageal lesions.

27. A 30-year-old man from rural Latin America presents with new-onset seizures. He reports frequent headaches in recent months. MRI of the brain reveals multiple cystic lesions, some of which have mural nodules (suggestive of scolex). The patient grew up in an area where pigs are raised and often consumed undercooked pork. Which of the following is the most likely cause of his seizures?

A. Toxoplasma gondii infection from cat feces
B. Taenia saginata (beef tapeworm) ingestion
C. Taenia solium (pork tapeworm) egg ingestion
D. Naegleria fowleri amoebic infection
E. Echinococcus granulosus hydatid cysts

Answer and Explanation: The correct answer is C. Taenia solium (pork tapeworm) egg ingestion. The described condition is neurocysticercosis, which is caused by the cystic larval stage of Taenia solium in the human brain​. Humans develop this by ingesting T. solium eggs (usually via food or water contaminated with feces from a tapeworm carrier). The larvae then disseminate and form cysts in various tissues, especially the brain, leading to seizures and characteristic cystic lesions on imaging. Toxoplasma gondii (choice A) can cause ring-enhancing lesions in the brain of immunocompromised patients, but in an immunocompetent person it usually causes asymptomatic infection or mild illness, not multiple cystic brain lesions with scolex. Taenia saginata (choice B) is the beef tapeworm; ingestion of its larvae in undercooked beef causes an intestinal tapeworm, but it does not cause cysticercosis in tissues. Naegleria fowleri (choice D) causes a fulminant, rapidly fatal meningoencephalitis (not chronic cysts) after swimming in warm freshwater. Echinococcus granulosus (choice E) causes hydatid cysts, typically in the liver (and sometimes lungs); brain involvement is less common and imaging would show a large hydatid cyst rather than multiple smaller cysticerci with scolex.

28. A 6-year-old child from an under-vaccinated community presents with sore throat, fever, and difficulty breathing. On examination, there is a thick gray-white patch covering the tonsils and pharynx that bleeds when attempted to remove. His neck is swollen. Which of the following is the most likely causative organism?

A. Corynebacterium diphtheriae
B. Streptococcus pyogenes
C. Epstein-Barr virus
D. Haemophilus influenzae type b
E. Candida albicans

Answer and Explanation: The correct answer is A. Corynebacterium diphtheriae. This child has diphtheria, characterized by a tough, gray pseudomembrane on the throat and "bull neck" lymphadenopathy​. C. diphtheriae produces a potent exotoxin that inactivates elongation factor-2, leading to local tissue necrosis (pseudomembrane) and potential systemic effects (heart and nerve damage). Vaccination (DTaP) can prevent this disease. Streptococcus pyogenes (choice B) causes strep throat, which can cause exudative pharyngitis but not a thick adherent gray pseudomembrane. Epstein-Barr virus (choice C) causes mononucleosis with throat exudates and lymphadenopathy, but typically also causes fatigue and hepatosplenomegaly, and it doesn’t form a gray membranous pharyngeal coating. Haemophilus influenzae type b (choice D) can cause epiglottitis in unvaccinated children (with drooling and airway obstruction), but it does not produce a pharyngeal pseudomembrane. Candida albicans (choice E) can cause oral thrush (white plaques in the mouth) in infants or immunocompromised patients, but thrush plaques are white, easily scraped off (unlike diphtheritic membrane), and usually not associated with systemic toxicity or airway obstruction.

29. A 17-year-old girl develops a pink maculopapular rash that started on her face and quickly spread to her trunk and extremities. She has a low-grade fever and complains of mild joint pains. On examination, you note tender posterior auricular and suboccipital lymphadenopathy. She has not received routine vaccinations. Which of the following is the most likely cause of her illness?

A. Rubella virus
B. Rubeola (measles) virus
C. Parvovirus B19
D. Human herpesvirus 6 (HHV-6)
E. Streptococcus pyogenes

Answer and Explanation: The correct answer is A. Rubella virus. Rubella (German measles) is a togavirus that causes a mild illness characterized by fever, lymphadenopathy, and a fine maculopapular rash that spreads from the face downward​publications.aap.org. Posterior auricular and suboccipital lymph node swelling is particularly associated with rubella infection. In adolescents and adults, rubella can also cause arthralgias. Measles (rubeola) (choice B) causes a more severe illness with higher fever, cough, coryza, conjunctivitis, Koplik spots, and a darker rash that usually lasts longer; lymphadenopathy in rubella is more pronounced and characteristic. Parvovirus B19 (choice C) causes erythema infectiosum (fifth disease) in children (slapped-cheek rash) and can cause arthralgias in adults, but it does not typically cause the diffuse rash with postauricular lymphadenopathy described here. Human herpesvirus 6 (choice D) causes roseola (exanthem subitum) in infants – high fever followed by a rash – and is not a cause of illness in teenagers. Streptococcus pyogenes (choice E) can cause scarlet fever (fever with a sandpaper-like rash), but scarlet fever is usually accompanied by pharyngitis and strawberry tongue, and it’s bacterial (the question describes a viral exanthem).

30. A 55-year-old man with acute leukemia undergoes intensive chemotherapy and develops neutropenia (very low neutrophil count). While hospitalized, he develops fever, cough, and hemoptysis. Chest CT shows several nodular lesions with surrounding ground-glass opacities ("halo sign"). A biopsy of a lung lesion reveals branched septate hyphae at a 45° angle invading the tissue. Which of the following is the most likely pathogen?

A. Pneumocystis jirovecii
B. Mucor and Rhizopus species
C. Candida albicans
D. Aspergillus fumigatus
E. Mycobacterium tuberculosis

Answer and Explanation: The correct answer is D. Aspergillus fumigatus. Invasive aspergillosis is a major risk in profoundly neutropenic patients. It typically presents with fever, cough, and hemoptysis, and imaging may show nodules with a surrounding "halo" of hemorrhage. Histologically, Aspergillus shows narrow, septate hyphae with acute angle (approximately 45°) branching​emcrit.org. Pneumocystis jirovecii (choice A) causes pneumonia in AIDS patients, typically with diffuse bilateral ground-glass infiltrates; it does not form hyphae visible on biopsy (it appears as disc-shaped yeast on methenamine silver stain). Mucor/Rhizopus (choice B) cause mucormycosis, often in diabetic ketoacidosis or neutropenia, but those molds have broad, ribbon-like non-septate hyphae with wide-angle (90°) branching. Candida albicans (choice C) can cause disseminated infection in neutropenic patients, but in the lungs it more commonly causes thrush or opportunistic pneumonia in immunocompromised; it is a yeast that forms pseudohyphae, not septate hyphae with acute-angle branching invading lung tissue. Mycobacterium tuberculosis (choice E) can cause hemoptysis and lung lesions (cavities), but it’s unlikely to cause an acute infection in a neutropenic hospitalized patient and does not show hyphae on biopsy.

31. A 45-year-old man from Brazil presents with difficulty swallowing (dysphagia) and chronic constipation. Imaging reveals a massively dilated esophagus and colon (megaesophagus and megacolon). He also has an enlarged heart on chest X-ray. He reports having been bitten by "kissing bugs" many years ago. Which of the following is the most likely cause of his condition?

A. Trypanosoma brucei
B. Entamoeba histolytica
C. Giardia lamblia
D. Clostridium botulinum
E. Trypanosoma cruzi

Answer and Explanation: The correct answer is E. Trypanosoma cruzi. T. cruzi is a protozoan parasite transmitted by the Reduviid (kissing) bug and is the cause of Chagas disease, which can lead to chronic destruction of autonomic ganglia. The classic manifestations include megacolon, megaesophagus, and a dilated cardiomyopathy years after the initial infection. Trypanosoma brucei (choice A) causes African sleeping sickness (tsetse fly transmission) characterized by recurring fevers and CNS involvement (somnolence), not GI tract dilation. Entamoeba histolytica (choice B) causes amebic dysentery and liver abscesses, not organ megasyndromes. Giardia lamblia (choice C) causes giardiasis (greasy diarrhea) from contaminated water and does not cause chronic organ enlargement. Clostridium botulinum (choice D) causes botulism (flaccid paralysis) via toxin, unrelated to the findings in this patient.

32. A 50-year-old man who works in a wool processing factory presents with sudden-onset shortness of breath, chest pain, and high fever. On exam, he appears acutely ill and is sweating and cyanotic. Chest X-ray reveals a widened mediastinum. Despite aggressive care, he dies within a day. An autopsy shows hemorrhagic mediastinitis. Which of the following is the most likely cause of his illness?

A. Yersinia pestis
B. Francisella tularensis
C. Coxiella burnetii
D. Bacillus anthracis
E. Histoplasma capsulatum

Answer and Explanation: The correct answer is D. Bacillus anthracis. Inhalational anthrax (woolsorter's disease) occurs from inhaling spores of B. anthracis (often from animal wool or hides). It causes a fulminant pulmonary infection with hemorrhagic mediastinitis, often evidenced by a widened mediastinum on imaging, and is frequently fatal. Yersinia pestis (choice A) causes plague; pneumonic plague can cause severe pneumonia, but it’s associated with rodent fleas and buboes (lymphadenopathy), not classically a widened mediastinum. Francisella tularensis (choice B) causes tularemia (from rabbits/ticks), which can lead to pneumonia, but the scenario and occupational exposure fit anthrax. Coxiella burnetii (choice C) causes Q fever (from farm animals), typically a flu-like illness or pneumonia without the dramatic mediastinal hemorrhage. Histoplasma capsulatum (choice E) is a fungus from bird/bat droppings that can cause lung infection and mediastinal lymphadenopathy, but an acute, rapidly fatal mediastinitis in a wool worker strongly points to inhalational anthrax.

33. Cells infected with a virus often release a substance that helps neighboring uninfected cells mount defenses to inhibit viral replication. For example, this substance induces nearby cells to produce antiviral proteins that degrade viral mRNA and impair protein synthesis. Which of the following mediators is primarily responsible for this effect?

A. Interleukin-1 (IL-1)
B. Interleukin-2 (IL-2)
C. Interleukin-10 (IL-10)
D. Interferon alpha and beta
E. Tumor necrosis factor-alpha (TNF-α)

Answer and Explanation: The correct answer is D. Interferon alpha and beta. Virus-infected cells secrete type I interferons (IFN-α and IFN-β) to signal neighboring cells​. These interferons bind to receptors on adjacent cells and induce an antiviral state: they trigger those cells to produce enzymes (like RNases and protein kinase R) that degrade viral mRNA and inhibit protein synthesis, thereby preventing viral replication if the neighboring cells become infected. IL-1 (choice A) causes fever and inflammation. IL-2 (choice B) stimulates growth of T cells and NK cells. IL-10 (choice C) is an anti-inflammatory cytokine that downregulates immune responses. TNF-α (choice E) mediates inflammation and can cause cachexia; it is involved in septic shock. None of those have the specific antiviral paracrine effect that interferons do.

34. A 40-year-old man with AIDS (CD4 count 30/µL) reports blurry vision and floaters in his right eye. Fundoscopic exam reveals perivascular hemorrhages and white, fluffy retinal lesions ("cotton wool" exudates). He is at risk of retinal detachment if left untreated. Which of the following is the most likely cause of his eye condition?

A. Toxoplasma gondii
B. Herpes simplex virus
C. Cryptococcus neoformans
D. JC virus (Polyomavirus)
E. Cytomegalovirus

Answer and Explanation: The correct answer is E. Cytomegalovirus. CMV retinitis is the most common opportunistic ocular infection in AIDS and a leading cause of blindness in these patients​webeye.ophth.uiowa.edu. It typically occurs when CD4 counts fall below 50/µL. Fundoscopy often shows retinal hemorrhages and fluffy or granular lesions (sometimes described as a "pizza pie" appearance). Toxoplasma gondii (choice A) can cause chorioretinitis, but in HIV it more commonly causes ring-enhancing brain lesions; its retinal lesions are typically more focal with severe inflammation. Herpes simplex virus (choice B) can cause acute retinal necrosis in immunocompetent or immunocompromised hosts, but CMV is far more common in AIDS. Cryptococcus neoformans (choice C) causes meningitis in AIDS, not retinitis. JC virus (choice D) causes progressive multifocal leukoencephalopathy (PML) – a demyelinating brain infection in AIDS – and does not infect the retina.

35. A 58-year-old man with long-term corticosteroid use for an autoimmune disease presents with weight loss, fever, and a chronic cough. Chest imaging reveals a cavitary lesion in the lung. Sputum cultures grow gram-positive branching filaments that weakly stain acid-fast. Brain MRI also shows a ring-enhancing abscess. Which of the following organisms is the most likely cause of this infection?

A. Actinomyces israelii
B. Nocardia asteroides
C. Mycobacterium tuberculosis
D. Streptococcus pneumoniae
E. Aspergillus fumigatus

Answer and Explanation: The correct answer is B. Nocardia asteroides. Nocardia is an aerobic, filamentous, branching gram-positive bacterium that is weakly acid-fast​. It tends to infect immunocompromised patients (e.g., on chronic steroids) and can cause chronic pulmonary infections with cavitation, often disseminating to form abscesses in the brain. Actinomyces israelii (choice A) also forms branching filaments, but it is anaerobic, not acid-fast, and is associated with cervicofacial abscesses (e.g., "lumpy jaw" with sulfur granules) in immunocompetent individuals after dental trauma. Mycobacterium tuberculosis (choice C) causes cavitary lung lesions and brain tuberculomas, but it is a rod (not branching filament) and strongly acid-fast; plus, the culture characteristics here point to Nocardia. Streptococcus pneumoniae (choice D) causes acute lobar pneumonia, not chronic cavitary lesions or brain abscesses in this context. Aspergillus fumigatus (choice E) is a fungus (not bacteria) that can cause cavitary lung lesions (e.g., fungus balls) in prior cavities or invasive disease in neutropenic patients, but it would show septate hyphae rather than acid-fast bacteria on culture.

36. A 3-year-old boy has had multiple severe infections since infancy, including abscesses caused by Staphylococcus aureus and Serratia marcescens. His neutrophils show an inability to generate respiratory burst products, and a nitroblue tetrazolium test is abnormal. Which of the following is the most likely underlying immunodeficiency?

A. A defect in NADPH oxidase enzyme in phagocytes
B. Myeloperoxidase deficiency in neutrophils
C. C1 esterase inhibitor deficiency
D. IL-2 receptor gamma chain mutation
E. Selective IgA immunoglobulin deficiency

Answer and Explanation: The correct answer is A. A defect in NADPH oxidase enzyme in phagocytes. This describes Chronic Granulomatous Disease (CGD), an X-linked (most often) immunodeficiency where phagocytes cannot generate reactive oxygen species due to a defective NADPH oxidase​. Patients with CGD have recurrent infections with catalase-positive organisms (like Staph aureus, Serratia, Nocardia, Aspergillus), because these microbes can neutralize their own H₂O₂ and phagocytes cannot compensate by generating ROS. The nitroblue tetrazolium (NBT) test fails to turn blue in CGD due to lack of superoxide production. Myeloperoxidase deficiency (choice B) also affects the oxidative killing pathway, but it is usually milder and often presents with Candida infections; the NBT test is normal in that condition. C1 esterase inhibitor deficiency (choice C) causes hereditary angioedema, not recurrent infections. IL-2 receptor gamma chain mutation (choice D) causes X-linked SCID, which leads to severe combined immunodeficiency (no B or T cell function), typically manifesting with all kinds of infections early in life, not specifically catalase-positive organisms. Selective IgA deficiency (choice E) causes mucosal infections (sinopulmonary, GI) but not the pattern of deep abscesses with catalase-positive bacteria seen in this case.

37. A 4-year-old boy from a rural area is brought to the hospital with abdominal distension and vomiting. His mother reports that he passed a large worm in his stool earlier. Imaging of the abdomen suggests intestinal obstruction in the small bowel. Which of the following parasites is the most likely cause of his symptoms?

A. Necator americanus
B. Enterobius vermicularis
C. Ascaris lumbricoides
D. Schistosoma mansoni
E. Trichuris trichiura

Answer and Explanation: The correct answer is C. Ascaris lumbricoides. Ascaris is the giant roundworm, often acquired via fecal-oral transmission of eggs in areas with poor sanitation. In heavy infestations, the adult worms (which can be up to 10-30 cm long) can form a mass and cause mechanical intestinal obstruction​, particularly in children. Passing a large worm in stool is classic for Ascaris infection. Necator americanus (choice A) is hookworm, which primarily causes anemia by sucking blood in the intestines, not typically bowel obstruction. Enterobius vermicularis (choice B) is the pinworm causing perianal itching; it is small and does not cause obstruction. Schistosoma mansoni (choice D) is a blood fluke that causes intestinal and liver schistosomiasis (fibrosis, portal hypertension), not acute bowel blockage by worms. Trichuris trichiura (choice E) is the whipworm, which can cause diarrhea and rectal prolapse in heavy infection, but not the classic nocturnal anal itching or a large worm bolus seen with Ascaris.

38. A 44-year-old man has epigastric pain that tends to be worse at night and improves after he eats a meal. An endoscopy reveals a duodenal ulcer. A rapid urease test on a biopsy sample is positive. Which of the following is the most likely underlying cause of his ulcer?

A. Helicobacter pylori infection
B. Campylobacter jejuni infection
C. Epstein-Barr virus infection
D. Clostridium difficile colonization
E. Cytomegalovirus infection

Answer and Explanation: The correct answer is A. Helicobacter pylori infection. H. pylori is a urease-positive spiral bacterium and is a major cause of peptic ulcer disease​, especially duodenal ulcers. A positive urease test (as in the biopsy or urea breath test) indicates the presence of H. pylori. Eradication of H. pylori with appropriate antibiotics and acid suppression is the typical treatment. Campylobacter jejuni (choice B) causes gastrointestinal infections (diarrhea), not peptic ulcers. Epstein-Barr virus (choice C) is associated with mononucleosis and some cancers (e.g., nasopharyngeal carcinoma), not duodenal ulcers. Clostridium difficile (choice D) causes antibiotic-associated colitis, not peptic ulcer disease. Cytomegalovirus (choice E) can cause ulcers in immunosuppressed patients (e.g., CMV esophagitis or colitis in AIDS), but duodenal ulcers in an otherwise healthy patient are most commonly due to H. pylori or NSAID use, with H. pylori indicated by the positive urease test.

39. A new influenza A strain is identified that contains a novel hemagglutinin protein derived from an avian influenza strain. Investigators determine that the virus arose from co-infection of a pig with human and avian influenza viruses, allowing mixing of their segmented RNA genomes. This process, responsible for pandemics, is best described as which of the following?

A. Antigenic drift due to point mutation
B. Antigenic drift due to RNA polymerase error
C. Phenotypic masking of viral particles
D. Recombination between viral HA and NA genes
E. Antigenic shift via genome segment reassortment

Answer and Explanation: The correct answer is E. Antigenic shift via genome segment reassortment. Influenza A viruses have a segmented RNA genome, and when two different strains infect the same cell (e.g., human and avian flu in a pig), they can exchange RNA segments (reassortment). This can result in a dramatically new hemagglutinin and/or neuraminidase combination​sciencedirect.com, leading to pandemics because the population lacks immunity. Antigenic drift (choices A and B) refers to small, gradual changes due to point mutations in viral genes (like HA or NA) over time – this causes seasonal flu changes, not sudden major shifts. Phenotypic masking (choice C) is when one virus is coated by surface proteins of another virus (not relevant here). Recombination (choice D) refers to crossing over between genes on the same segment of two viruses; influenza’s major changes are usually due to reassortment of whole segments (shift) rather than intrasegment recombination.

40. A 30-year-old man from Ohio visits his physician for chronic cough and fatigue. He reports that he is a cave explorer (spelunker). Chest X-ray shows hilar lymphadenopathy and scattered calcifications in the lungs. A biopsy of a lung lesion reveals granulomas with small yeasts inside macrophages. Which of the following is the most likely cause of his infection?

A. Coccidioides immitis
B. Blastomyces dermatitidis
C. Mycobacterium tuberculosis
D. Histoplasma capsulatum
E. Cryptococcus neoformans

Answer and Explanation: The correct answer is D. Histoplasma capsulatum. Histoplasmosis is endemic in the Ohio and Mississippi River valleys and is associated with exposure to bat or bird droppings (such as in caves). The small yeasts of Histoplasma are often found within macrophages, and infection can cause granulomas in the lungs and a clinical picture that mimics tuberculosis (including hilar adenopathy and calcified lesions). Coccidioides immitis (choice A) is found in the Southwest US (desert areas) and forms spherules filled with endospores in tissue. Blastomyces dermatitidis (choice B) is found in the Eastern US (including Ohio/Mississippi regions too) and is a broad-based budding yeast, but it typically causes pneumonia with skin lesions or bone involvement; the association with bat caves and intracellular yeasts points to Histoplasma. Mycobacterium tuberculosis (choice C) causes granulomas and calcified lesions (Ghon complexes) in the lung, but the yeast forms in macrophages and exposure history favor Histoplasma. Cryptococcus neoformans (choice E) is associated with pigeon droppings and primarily causes meningitis in immunocompromised patients, not granulomatous lung disease in healthy spelunkers.

41. A 10-year-old boy in North Carolina develops high fever, headache, and a rash that started on his wrists and ankles and is now spreading to his trunk. His parents recall removing several ticks from his skin after a camping trip a week ago. Which of the following is the most likely cause of his illness?

A. Borrelia burgdorferi
B. Ehrlichia chaffeensis
C. Coxiella burnetii
D. Rickettsia rickettsii
E. Treponema pallidum

Answer and Explanation: The correct answer is D. Rickettsia rickettsii. This is the agent of Rocky Mountain Spotted Fever, transmitted by Dermacentor ticks in the Southeastern US. It causes a vasculitic rash that classically begins on the wrists and ankles and spreads centrally, accompanied by fever and headache. Borrelia burgdorferi (choice A) causes Lyme disease (erythema migrans rash and later arthritic/neurologic symptoms). Ehrlichia chaffeensis (choice B) causes Ehrlichiosis (lone star tick, with fever and monocyte inclusions, usually without rash). Coxiella burnetii (choice C) causes Q fever (from inhaling farmyard dust), which presents with fever and pneumonia but no rash. Treponema pallidum (choice E) causes syphilis; secondary syphilis can cause a rash on palms and soles, but the scenario and tick exposure fit RMSF.

42. A 32-year-old woman has an abnormal Pap smear showing dysplastic cells and testing positive for DNA of a sexually transmitted virus. The virus is non-enveloped, has a circular double-stranded DNA genome, and is strongly linked to cervical cancer. Which of the following is the most likely cause of her cervical dysplasia?

A. Epstein-Barr virus (EBV)
B. Human papillomavirus (HPV) type 16
C. Human herpesvirus 8 (HHV-8)
D. Hepatitis B virus (HBV)
E. Human T-lymphotropic virus (HTLV-1)

Answer and Explanation: The correct answer is B. Human papillomavirus (HPV) type 16. High-risk HPV strains (especially types 16 and 18) produce oncogenic proteins (E6 and E7) that inactivate tumor suppressors (p53 and Rb), leading to cervical intraepithelial neoplasia and carcinoma. The Pap smear and DNA test implicate HPV in this patient's cervical dysplasia. Epstein-Barr virus (choice A) is associated with nasopharyngeal carcinoma and certain lymphomas, not cervical cancer. HHV-8 (choice C) is linked to Kaposi sarcoma. Hepatitis B (choice D) is associated with hepatocellular carcinoma. HTLV-1 (choice E) is linked to adult T-cell leukemia/lymphoma.

43. A patient with poorly controlled diabetes mellitus develops acute sinus pain, fever, and black necrotic lesions on his nasal turbinates. Biopsy of the lesion shows broad non-septate hyphae with right-angle branching. Which of the following organisms is the most likely cause?

A. Aspergillus fumigatus
B. Nocardia asteroides
C. Staphylococcus aureus
D. Actinomyces israelii
E. Mucor or Rhizopus species

Answer and Explanation: The correct answer is E. Mucor or Rhizopus species. This presentation is classic for mucormycosis in a diabetic patient (often in ketoacidosis). These fungi have broad, ribbon-like non-septate hyphae that branch at 90° angles and invade blood vessels, causing necrosis of nasal and sinus tissue. Aspergillus fumigatus (choice A) has septate hyphae with acute angle (45°) branching and typically causes lung infections or fungus balls, not the fulminant sinus infection described. Nocardia (choice B) is a filamentous bacterium affecting lungs/brain in immunocompromised, not typically the sinuses. Staph aureus (choice C) can cause sinusitis but would not show fungal hyphae on biopsy. Actinomyces (choice D) causes cervicofacial abscesses with sulfur granules, not acute necrotic sinusitis.

44. A newborn baby girl develops signs of meningitis (fever, irritability, poor feeding) at 2 weeks of age. Culture of the cerebrospinal fluid yields beta-hemolytic, gram-positive cocci in chains that are bacitracin resistant. The mother’s prenatal records show no documented screening or prophylaxis for this organism. Which of the following is the most likely cause of the baby's infection?

A. Escherichia coli
B. Listeria monocytogenes
C. Streptococcus pneumoniae
D. Neisseria meningitidis
E. Streptococcus agalactiae

Answer and Explanation: The correct answer is E. Streptococcus agalactiae (Group B Streptococcus). Group B Strep is a leading cause of neonatal meningitis, typically acquired from the mother during birth. It is a beta-hemolytic gram-positive cocci in chains that is bacitracin resistant and CAMP test positive. Pregnant women are screened for GBS late in pregnancy and given intrapartum penicillin if positive to prevent newborn infection. Escherichia coli (choice A) (with the K1 capsule) is another common neonatal meningitis cause, but it is a gram-negative rod, not a gram-positive cocci. Listeria (choice B) can cause neonatal meningitis, but it’s a motile gram-positive rod often associated with contaminated food or transplacental transmission, and the culture here indicated cocci. Streptococcus pneumoniae (choice C) causes meningitis in older infants and adults, not typically in a neonate. Neisseria meningitidis (choice D) causes meningitis in infants, children, and young adults, and is a gram-negative diplococcus.

45. A 25-year-old man with sickle cell disease presents with fatigue and pallor. His hemoglobin is much lower than his baseline and his reticulocyte count is nearly zero, indicating an aplastic crisis. Which of the following infectious agents is most likely responsible?

A. Epstein-Barr virus
B. Parvovirus B19
C. Salmonella enterica (serotype Osteomyelitis)
D. Folate deficiency
E. Babesia microti

Answer and Explanation: The correct answer is B. Parvovirus B19. Parvovirus B19 infects erythroid precursors in the bone marrow and can cause an "aplastic crisis" in patients with chronic hemolytic anemias like sickle cell disease. These patients have a high turnover of RBCs and rely on a robust reticulocyte production, which Parvovirus temporarily shuts down. Epstein-Barr virus (choice A) can cause aplastic anemia in rare cases, but it’s more known for mononucleosis; Parvovirus is the classic cause of transient aplastic crisis in hemolytic anemia patients. Salmonella (choice C) is a common cause of osteomyelitis in sickle cell patients, not aplastic crisis. Folate deficiency (choice D) can cause anemia, but not an acute shut-down of erythropoiesis with low reticulocytes in a sickle cell patient (and it's not an infectious agent). Babesia microti (choice E) causes hemolysis (babesiosis) and can worsen anemia in asplenic individuals (like many sickle cell patients), but it does not directly cause an aplastic crisis.

46. A 22-year-old man presents with urethral discharge and dysuria. He is sexually active and does not regularly use condoms. Gram stain of the purulent discharge reveals many neutrophils with intracellular gram-negative diplococci. Which of the following is the most likely cause of his symptoms?

A. Chlamydia trachomatis serotype D-K
B. Neisseria gonorrhoeae
C. Ureaplasma urealyticum
D. Escherichia coli
E. Mycoplasma genitalium

Answer and Explanation: The correct answer is B. Neisseria gonorrhoeae. Gonorrhea in men classically presents with a purulent urethral discharge and dysuria. Gram-negative intracellular diplococci on Gram stain of the exudate confirm the diagnosis of gonococcal urethritis. Chlamydia trachomatis (choice A) can cause non-gonococcal urethritis (often with a more watery discharge), but Chlamydia are not visible on Gram stain (they are intracellular and lack a typical peptidoglycan cell wall). Ureaplasma and Mycoplasma (choices C and E) can cause urethritis as well, but they also would not show up as Gram-negative diplococci on stain. E. coli (choice D) is a common cause of urinary tract infections, but in sexually active young men with urethritis and a purulent discharge, a sexually transmitted pathogen is indicated (and E. coli would be a rod, not a diplococcus).

47. A 60-year-old man reports severe pain in a band on the left side of his chest, followed by the eruption of clusters of vesicular lesions in the same dermatomal distribution. He had chickenpox as a child. Which of the following is the most likely cause of his current condition?

A. Herpes simplex virus type 1
B. Varicella-zoster virus
C. Coxsackie A virus
D. Staphylococcus aureus
E. Borrelia burgdorferi

Answer and Explanation: The correct answer is B. Varicella-zoster virus (VZV). The reactivation of latent VZV in dorsal root ganglia leads to herpes zoster (shingles), characterized by a painful vesicular rash in a dermatomal distribution. This patient’s history of childhood chickenpox (primary VZV infection) and the clinical scenario point to shingles. Herpes simplex virus-1 (choice A) can cause grouped vesicles, but typically around the mouth or face (cold sores) and not usually strictly dermatomal. Coxsackie A virus (choice C) causes hand-foot-and-mouth disease and herpangina, not a dermatomal vesicular rash in adults. Staphylococcus aureus (choice D) can cause skin infections, but vesicles in a dermatomal pattern with preceding neuropathic pain is classic for shingles. Borrelia burgdorferi (choice E) causes Lyme disease (erythema migrans, not vesicular rash).

48. A 30-year-old man sustains a deep, dirty puncture wound in his leg in a motorcycle accident. Two days later, he develops severe pain, swelling, and crepitus (gas) in the wound area, with a foul-smelling brown discharge. Which of the following is the most likely causative organism?

A. Clostridioides difficile
B. Clostridium tetani
C. Clostridium perfringens
D. Pasteurella multocida
E. Streptococcus pyogenes

Answer and Explanation: The correct answer is C. Clostridium perfringens. C. perfringens is an anaerobic, spore-forming gram-positive rod that causes gas gangrene (clostridial myonecrosis). It often infects deep wounds with lots of tissue necrosis and poor oxygenation. It produces gas (hence crepitus) and foul-smelling anaerobic wound discharge, and its alpha toxin causes rapid tissue destruction. Clostridioides difficile (choice A) causes pseudomembranous colitis, not wound infections. Clostridium tetani (choice B) can infect wounds but causes tetanus (muscle spasms) rather than gas gangrene, and it typically does not produce significant tissue gas or necrosis at the wound site. Pasteurella multocida (choice D) causes wound infections from animal bites (especially cats or dogs) with rapid cellulitis, but not gas gangrene. Streptococcus pyogenes (choice E) can cause necrotizing fasciitis, which can be severe and malodorous, but the prominent gas production points more toward Clostridial infection.

49. An infant who has not received immunizations develops a severe cough. The episodes consist of multiple bursts of coughing on a single exhalation, followed by a loud inspiration. During these coughing spells, the infant becomes cyanotic and vomits. Which of the following is the most likely cause of this illness?

A. Respiratory syncytial virus (RSV)
B. Parainfluenza virus (croup)
C. Chlamydia trachomatis D-K
D. Mycoplasma pneumoniae
E. Bordetella pertussis

Answer and Explanation: The correct answer is E. Bordetella pertussis. This is classic whooping cough – paroxysmal coughs followed by a "whoop" inhalation and often post-tussive vomiting in an unvaccinated infant. B. pertussis releases toxins that damage the respiratory epithelium and cause lymphocytosis. RSV (choice A) causes bronchiolitis in infants (wheezing, cough, difficulty breathing) but not the distinctive whooping cough pattern. Parainfluenza virus (choice B) causes croup (barking cough and stridor) in young children, different from pertussis. Chlamydia trachomatis (choice C) can cause a staccato cough and pneumonia in infants (usually 1-3 months old) born to infected mothers, but not the whooping cough pattern. Mycoplasma pneumoniae (choice D) causes atypical pneumonia, mostly in school-age children or adults, not this infant’s presentation.

50. A 5-year-old girl has intense perianal itching, especially at night. No other symptoms are noted. Scotch tape applied to the perianal region in the morning shows oval eggs. Which of the following is the most likely causative organism?

A. Strongyloides stercoralis
B. Taenia solium
C. Trichomonas vaginalis
D. Trichuris trichiura
E. Enterobius vermicularis

Answer and Explanation: The correct answer is E. Enterobius vermicularis. Enterobius, or pinworm, is a common helminth in children. It causes perianal itching due to female worms migrating to lay eggs around the anus at night. The "tape test" detecting eggs confirms the diagnosis. Strongyloides stercoralis (choice A) causes gastrointestinal issues (and can autoinfect), not primarily perianal itching. Taenia solium (choice B) is the pork tapeworm (causes cysticercosis if eggs are ingested, or an intestinal tapeworm if larvae are ingested) – it does not cause perianal itching. Trichomonas vaginalis (choice C) is a protozoan causing vaginal infection, not relevant here. Trichuris trichiura (choice D), the whipworm, can cause GI upset and rectal prolapse in heavy infections, but not the classic nocturnal anal itching.

51. A 34-year-old man presents with a wound infection on his hand. He was bitten on the hand by a cat, and within 24 hours the wound became red, swollen, and painful with purulent drainage. Which of the following organisms is the most likely cause?

A. Pasteurella multocida
B. Bartonella henselae
C. Streptobacillus moniliformis
D. Francisella tularensis
E. Capnocytophaga canimorsus

Answer and Explanation: The correct answer is A. Pasteurella multocida. Pasteurella is commonly found in the mouths of cats (and dogs) and causes rapidly developing wound infections with intense inflammation and purulence after bites or scratches. Cellulitis can develop within 24 hours. Bartonella henselae (choice B) is transmitted by cat scratches but causes cat-scratch disease (chronic lymphadenopathy) rather than an acute wound infection. Streptobacillus moniliformis (choice C) causes rat-bite fever, not typically associated with cats. Francisella tularensis (choice D) is associated with rabbit exposure or tick bites (tularemia). Capnocytophaga canimorsus (choice E) is another dog (and sometimes cat) bite organism, but it typically causes severe sepsis in asplenic individuals, whereas Pasteurella is the classic cause of rapidly progressive bite wound infections.

52. A 23-year-old woman is evaluated for a mucopurulent cervical discharge and cervical inflammation on exam. Nucleic acid amplification testing of the discharge is positive for an obligate intracellular organism that lacks a classic peptidoglycan cell wall. Which of the following is the most likely cause of this infection?

A. Neisseria gonorrhoeae
B. Chlamydia trachomatis (serotypes D–K)
C. Trichomonas vaginalis
D. Gardnerella vaginalis
E. Herpes simplex virus type 2

Answer and Explanation: The correct answer is B. Chlamydia trachomatis, serotypes D–K. These serotypes of Chlamydia cause the most common bacterial STD (cervicitis, urethritis, pelvic inflammatory disease). Chlamydia are obligate intracellular bacteria (cannot make their own ATP efficiently) and have a unique cell wall without classic peptidoglycan (hence beta-lactams are less effective). Nucleic acid amplification (PCR) is the test of choice. Neisseria gonorrhoeae (choice A) also causes a purulent cervicitis, but it’s a gram-negative diplococcus with a typical cell wall and is usually identified by Gram stain/culture or PCR separately; coinfection with Chlamydia is common. Trichomonas vaginalis (choice C) causes a frothy green vaginal discharge and vaginitis, not just mucopurulent cervicitis. Gardnerella vaginalis (choice D) causes bacterial vaginosis (thin gray discharge with fishy odor), not cervicitis. HSV-2 (choice E) causes genital ulcers, not a mucopurulent discharge.

53. A hunter in Alaska develops fever, muscle pains, and swelling around the eyes (periorbital edema) about two weeks after eating undercooked bear meat. Blood tests show marked eosinophilia. Which of the following parasites is the most likely cause of his symptoms?

A. Taenia solium
B. Trypanosoma cruzi
C. Toxocara canis
D. Taenia saginata
E. Trichinella spiralis

Answer and Explanation: The correct answer is E. Trichinella spiralis. Trichinella is acquired by eating undercooked meat (often pork or wild game like bear) containing encysted larvae. The larvae migrate to skeletal muscle, causing muscle pain and inflammation; periorbital edema and high eosinophil counts are characteristic. Taenia solium (choice A) is the pork tapeworm; ingestion of larvae leads to an intestinal tapeworm, and ingestion of eggs leads to cysticercosis (brain cysts), not the described syndrome. Trypanosoma cruzi (choice B) causes Chagas disease (cardiomyopathy, megacolon, etc.), unrelated to eating bear meat. Toxocara canis (choice C) causes visceral larva migrans from ingesting dog roundworm eggs (often in children), leading to organ inflammation and eosinophilia, but not specifically the muscle and eye findings noted here. Taenia saginata (choice D) is the beef tapeworm, which causes an intestinal infection from undercooked beef but doesn’t invade tissues or cause eosinophilia like Trichinella.

54. A 38-year-old woman vacationing in Connecticut notices a red, expanding rash on her thigh that resembles a “bull’s-eye.” She recalls removing a tick from that area of skin about a week ago. If left untreated, which of the following is the most likely causative organism that could later lead to arthritis and neurological symptoms in this patient?

A. Rickettsia rickettsii
B. Ehrlichia chaffeensis
C. Babesia microti
D. Borrelia burgdorferi
E. Dermacentor variabilis

Answer and Explanation: The correct answer is D. Borrelia burgdorferi. This spirochete causes Lyme disease, transmitted by Ixodes ticks in the Northeast US. The early localized stage features erythema migrans (the “target” rash). If untreated, later stages can include migratory arthritis, facial nerve palsy, and other neurologic or cardiac manifestations. Rickettsia rickettsii (choice A) causes Rocky Mountain Spotted Fever (rash that starts on wrists/ankles). Ehrlichia (choice B) causes ehrlichiosis (no rash in most cases, and it’s in the Southeast/central US). Babesia microti (choice C) is transmitted by the same tick and region, but it causes hemolytic anemia (babesiosis) rather than rash and arthritis. Dermacentor variabilis (choice E) is a tick (American dog tick) – a vector for RMSF and tularemia, not the cause of Lyme (and the question asks for the organism, not the vector).

55. A 27-year-old shepherd from an area endemic to sheep farming presents with a 10 cm cyst in his liver on imaging. The cyst has an “eggshell” calcified wall. Serology is positive for antibodies against a parasite. The physician advises caution during surgical removal of the cyst to prevent spilling its contents and causing an anaphylactic reaction. Which parasite is the most likely cause of this liver cyst?

A. Echinococcus granulosus
B. Entamoeba histolytica
C. Schistosoma mansoni
D. Clonorchis sinensis
E. Fasciola hepatica

Answer and Explanation: The correct answer is A. Echinococcus granulosus. This dog tapeworm causes hydatid cysts in humans (incidental intermediate hosts) who ingest eggs from dog feces. Sheep are part of its lifecycle, and sheep farmers are at risk. Hydatid cysts (often in liver) have a calcified wall and contain fluid with daughter cysts; spillage can cause a life-threatening anaphylactic reaction. Entamoeba histolytica (choice B) causes amebic liver abscesses (usually not calcified and described as “anchovy paste” pus, not large cysts). Schistosoma mansoni (choice C) causes periportal fibrosis and portal hypertension, not a discrete cystic lesion. Clonorchis sinensis (choice D), the Chinese liver fluke, infects bile ducts and is associated with cholangiocarcinoma, not large hydatid cysts. Fasciola hepatica (choice E), the liver fluke, can infect the liver/bile ducts from aquatic plants ingestion, but it causes more biliary inflammation rather than a huge cystic structure.