Metronidazole requires anaerobic activation to exert its bactericidal effect. The activated intermediate responsible for DNA damage is:

• A Metronidazole's nitro group is reduced by anaerobic electron carriers (ferredoxin, flavodoxin) to nitro-radical anions and hydroxylamine derivatives that form covalent adducts with DNA, causing strand breaks ✓
• B Metronidazole is reduced to a hydroxylamine by nitroreductases, which inhibits DNA gyrase
• C Metronidazole is activated by cytochrome P450 to an epoxide that alkylates guanine residues in DNA
• D Metronidazole generates superoxide radicals that oxidize thymidine residues causing G-C to A-T transversion mutations

Explanation

Metronidazole is a prodrug requiring intracellular activation in anaerobic organisms. Low-potential electron carriers in the electron transport chain of anaerobes (ferredoxin in bacteria and hydrogenosomes/mitochondria of parasites) donate electrons to the nitro group (-NO2) of metronidazole, generating nitro-radical anions (one-electron reduction), which further reduce to reactive metabolites including hydroxylamine derivatives. These intermediates form covalent adducts with DNA bases, cause DNA strand breaks, and disrupt helical structure, leading to cell death. Aerobic organisms cannot activate metronidazole (electrons from O2 instead reoxidize the radical anion — the 'futile cycle'), explaining the selectivity for anaerobes and microaerophilic organisms.

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

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