Metronidazole is selectively toxic to anaerobic organisms and microaerophilic protozoa because:
- A It binds only the ferredoxin-type electron transport proteins present exclusively in anaerobes
- B Its nitro group is reductively activated to cytotoxic radical intermediates only in low-oxygen environments by anaerobic/reduced ferredoxin systems ✓
- C Aerobic bacteria lack the outer membrane porins needed for metronidazole uptake
- D Metronidazole inhibits aerobic respiration selectively in organisms with complex I of the electron transport chain
Explanation
Metronidazole is a prodrug; its nitro group must be reduced to hydroxylamine radical intermediates by low-redox-potential electron carriers (ferredoxin, NADH-nitroreductase, pyruvate-ferredoxin oxidoreductase) found in anaerobes and microaerophilic organisms (Giardia, Trichomonas, Entamoeba). These radicals cause single-strand DNA breaks. In aerobic cells, oxygen instantly re-oxidises any reduced intermediates before they can damage DNA, providing selectivity. The answer A is close but imprecise — metronidazole's selectivity is about the reducing environment, not binding specificity.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.