A) Ganciclovir – inhibits DNA polymerase after phosphorylation by viral kinase B) Acyclovir – requires viral thymidine kinase for activation C) Foscarnet – directly inhibits viral DNA polymerase without prior phosphorylation D) Cidofovir – incorporates into DNA after diphosphate conversion E) Brivudine – inhibits viral thymidine kinase Answer: C – VZV retinitis in advanced HIV can be acyclovir-resistant due to thymidine kinase mutations. Foscarnet does not require viral TK; it directly blocks DNA polymerase. Acyclovir (B) would fail if TK-deficient. Ganciclovir (A) requires viral kinase (UL97 for CMV; VZV TK less efficient). Brivudine (E) also requires TK. 3. Gram-Positive Cocci – Subtle Differentiation A blood culture from a patient with subacute bacterial endocarditis grows catalase-negative, gram-positive cocci in chains. The organism is bile-esculin positive, grows in 6.5% NaCl, and produces a yellow pigment on blood agar. Which additional test confirms the species, and what is the key virulence factor?
A) HSV-1 – trigeminal ganglia – sunlight/UV B) VZV – dorsal root ganglia – emotional stress C) EBV – B lymphocytes – plasmapheresis D) CMV – salivary gland endothelial cells – trauma E) HHV-6 – microglia – rituximab therapy Answer: A – HSV-1 reactivation is classically triggered by UV light, fever, stress. VZV reactivation (shingles) often has no clear trigger but can be stress/age-related; sunlight is not classic. EBV latency in B cells; reactivation more with immunosuppression. CMV latency in monocytes; plasmapheresis not a trigger. HHV-6 in microglia; reactivation post-transplant, not rituximab specifically. 8. Spirochetes – Diagnostic Pitfall A patient with a painless genital ulcer and inguinal lymphadenopathy has a darkfield microscopy positive for spirochetes. However, the RPR is negative. Which of the following best explains this seronegative primary syphilis?
A) PYR positive – fibrinogen binding protein B) Optochin resistant – pneumolysin C) Bacitracin sensitive – M protein D) Hippurate hydrolysis – CAMP factor E) Lancefield group D antigen – cytolysin Answer: A – The organism is Enterococcus faecalis (bile-esculin +, 6.5% NaCl +). Among enterococci, E. faecalis is PYR positive (distinguishes from E. faecium sometimes). Key virulence for endocarditis includes aggregation substance and gelatinase. Option A’s “fibrinogen binding protein” refers to microbial surface component recognizing adhesive matrix molecules (MSCRAMMs). CAMP factor is Strep. agalactiae . Hippurate hydrolysis is S. agalactiae . 4. Anaerobes – Deep Concept A diabetic foot ulcer culture grows foul-smelling, gram-negative bacilli, resistant to kanamycin and vancomycin, but sensitive to metronidazole. Which enzyme system is directly inhibited by metronidazole in this organism? jawetz microbiology mcq
A) Lipid A-mediated cytokine storm B) IgA protease secretion C) Polysaccharide capsule that inhibits complement deposition D) Exotoxin A-mediated ADP-ribosylation of EF-2 E) M protein-mediated antiphagocytosis Answer: C – The organism is Haemophilus influenzae type b (requires X and V factors). Its polyribosylribitol phosphate (PRP) capsule is the major virulence factor for invasive disease (meningitis, epiglottitis). IgA protease (B) facilitates mucosal colonization but not invasion. Exotoxin A is from Pseudomonas . M protein is from Strep. pyogenes . 2. Antiviral Pharmacology A patient with HIV (CD4 count 180) on tenofovir, emtricitabine, and dolutegravir develops progressive outer retinal necrosis. PCR of vitreous fluid is positive for varicella-zoster virus (VZV). Which drug added to current ART would be most appropriate, and what is its mechanism?
A 45-year-old with a history of recurrent otitis media develops meningitis. CSF Gram stain shows small pleomorphic Gram-negative rods. The isolate requires X and V factors for growth. Which of the following virulence mechanisms is most directly associated with this organism’s ability to cause invasive disease? Ganciclovir (A) requires viral kinase (UL97 for CMV;
A) Mycolic acid chain length – Mycobacterium marinum B) Lipoarabinomannan (LAM) structure – Mycobacterium kansasii C) Phthiocerol dimycocerosate (PDIM) – Mycobacterium leprae D) Sulfolipids – Mycobacterium tuberculosis E) Phenolic glycolipids – Mycobacterium ulcerans Answer: A – M. marinum causes fish tank granuloma, grows optimally at 30-32°C, not at 37°C. Mycolic acid chain composition affects membrane fluidity. M. leprae (C) does not grow on artificial media. PDIM is important for M. tuberculosis virulence but not temperature restriction. 10. Mixed Infection – Synergy A human bite wound becomes necrotizing within 24 hours. Gram stain shows mixed pleomorphic gram-negative rods and tiny gram-positive cocci in chains. The infection is more severe than either isolate alone. Which pair of organisms and their synergistic virulence mechanism is correct?
A) Prozone phenomenon due to high antibody titer B) Infection with Treponema pallidum subspecies endemicum C) Simultaneous HIV infection causing B-cell dysfunction D) Early chancre (less than 1-2 weeks duration) E) Prior treatment with azithromycin Answer: D – RPR (nontreponemal) becomes positive 1–2 weeks after chancre appears. Very early primary syphilis can be RPR-negative but darkfield-positive. Prozone (A) occurs with high antibody titers causing false negative in undiluted serum, but usually in secondary syphilis. HIV (C) can cause false negatives or delayed seroreactivity, but the classic teaching is “too early.” 9. Mycobacteria – Cell Wall Function A patient with cavitary lung disease has an acid-fast bacillus that fails to grow on Lowenstein-Jensen medium at 37°C but grows rapidly at 30°C on Middlebrook 7H11. Which cell wall component accounts for this temperature restriction, and what is the organism? not typically rapid necrosis. B.
A) Flucytosine – inhibits thymidylate synthase B) Amphotericin B – binds ergosterol C) Caspofungin – inhibits β-(1,3)-D-glucan synthase D) Voriconazole – inhibits lanosterol 14α-demethylase E) Terbinafine – inhibits squalene epoxidase Answer: C – The description matches Aspergillus fumigatus (galactomannan +, green colony with red reverse). Echinocandins (caspofungin) target β-glucan, which is abundant in Aspergillus cell wall. Although voriconazole is drug of choice for invasive aspergillosis, the question asks for mechanism “specifically suited” to a unique cell wall component – β-glucan is more specific to fungal cell wall (not in human cells). Amphotericin B (B) targets ergosterol but also binds cholesterol, less specific. 6. Parasitology – Relapse Mechanism A returned traveler from Southeast Asia had 3 days of fever, chills, and sweats every 48 hours, now asymptomatic without treatment. Six months later, he develops identical symptoms. Which structure of Plasmodium vivax is responsible for this pattern, and what is its unique metabolic feature?
A) Trophozoite – glycolysis via hexose monophosphate shunt B) Schizont – proteolysis of hemoglobin C) Hypnozoite – dormant stage in hepatocytes with slow metabolic rate D) Gametocyte – anaerobic respiration E) Merozoite – pentose phosphate pathway only Answer: C – P. vivax and P. ovale form hypnozoites in the liver, causing relapse months after primary infection. They are metabolically dormant but survive. Primaquine targets them. The 48-hour periodicity is tertian malaria. 7. Virology – Unusual Latency Which DNA virus is correctly paired with its primary site of latency AND a unique reactivation trigger that does NOT involve immunosuppression?
A) Superoxide dismutase B) Catalase C) Pyruvate-ferredoxin oxidoreductase D) Cytochrome c oxidase E) Beta-lactamase Answer: C – The organism is Bacteroides fragilis group. Metronidazole is a prodrug reduced by ferredoxin (or pyruvate-ferredoxin oxidoreductase) in anaerobic bacteria; the reduced form damages DNA. Resistance can occur via nim genes that reduce metronidazole to inactive metabolites. Option A (SOD) is present in aerotolerant anaerobes but not metronidazole target. 5. Mycology – Antifungal Mechanism A patient with prolonged neutropenia develops a pulmonary cavity. A serum galactomannan antigen is positive. The isolate grows a greenish-brown colony with a red reverse on Sabouraud dextrose agar. Which drug’s mechanism is most specifically suited for this organism’s unique cell wall component?
A) Eikenella corrodens + Staphylococcus aureus – beta-lactamase protects both B) Fusobacterium nucleatum + Streptococcus anginosus – succinic acid and short-chain fatty acids inhibit phagocyte function C) Prevotella melaninogenica + Peptostreptococcus – hyaluronidase and collagenase D) Capnocytophaga + Streptococcus mitis – endotoxin synergy E) Bacteroides fragilis + Enterococcus faecalis – capsule and superoxide dismutase Answer: B – Fusobacterium + Streptococcus (especially S. anginosus group) is classic synergistic necrotizing infection (e.g., Lemierre’s, human bite). Fusobacterium produces succinic acid and short-chain fatty acids that impair neutrophil killing. Eikenella (A) is slow-growing, not typically rapid necrosis. B. fragilis + Enterococcus seen in intra-abdominal but not rapid 24h necrosis from human bite.