A 45-year-old patient with Clostridium difficile infection fails metronidazole therapy and is switched to oral vancomycin. Metronidazole's mechanism of action against anaerobes like C. difficile involves which key activation step?

• A Metronidazole inhibits RNA polymerase in C. difficile, preventing spore transcription factor synthesis
• B Metronidazole is activated by flavodoxin reductase and ferredoxin in anaerobic organisms, generating radical anion intermediates that cause single-strand DNA breaks and cytotoxic adducts ✓
• C Metronidazole is a prodrug cleaved by beta-lactamase in the anaerobic environment to produce the active nitroso metabolite
• D Metronidazole directly alkylates the electron transport chain complex I (NADH dehydrogenase) in the anaerobic bacterial membrane

Explanation

Metronidazole is a prodrug that requires reductive bioactivation specific to anaerobic and microaerophilic organisms. The nitro group is reduced by flavodoxin reductase (and pyruvate:ferredoxin oxidoreductase pathway) in anaerobes; this generates radical anion intermediates and cytotoxic hydroxylamine/nitroso derivatives that form adducts with DNA, causing single-strand breaks and inhibiting DNA strand repair. Aerobic organisms cannot reduce the nitro group (due to low cellular redox potential) and are thus spared — explaining metronidazole's selective anaerobic spectrum. It is not activated by beta-lactamase and does not inhibit RNA polymerase.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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