Jawetz Microbiology Mcq -

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) 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.

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) 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? Ganciclovir (A) requires viral kinase (UL97 for CMV;

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?

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? marinum causes fish tank granuloma

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?