Metronidazole is effective against anaerobic bacteria and protozoa (Giardia, Trichomonas, Entamoeba). Its selective toxicity to anaerobes is because:

• A The nitro group is reduced by anaerobic electron carriers (ferredoxin) to cytotoxic reactive intermediates only in low-redox-potential organisms ✓
• B Anaerobic organisms lack catalase to neutralize metronidazole-generated hydrogen peroxide
• C Metronidazole is activated by flavoprotein reductase present only in organisms lacking mitochondria
• D Anaerobes transport metronidazole against a concentration gradient using ABCG2 efflux pumps in reverse

Explanation

Metronidazole is a prodrug that is selectively activated in organisms with a sufficiently low intracellular redox potential. Anaerobic bacteria and protozoa (which have ferredoxin or ferredoxin-like electron transport proteins) reduce the nitro group of metronidazole to a short-lived nitroso radical intermediate. This reactive species causes strand breaks in DNA and disrupts its helical structure, leading to cell death. Aerobic cells cannot reduce metronidazole at physiological oxygen tensions because the high redox potential prevents ferredoxin from donating electrons to the drug—explaining selective activity. Resistance arises from reduced ferredoxin activity or increased DNA repair.

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

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