A 35-year-old woman presents with severe watery diarrhea after a course of clindamycin. Stool PCR confirms Clostridioides difficile toxin. She is started on oral metronidazole 500 mg thrice daily. Metronidazole acts as a tissue amebicide and is effective against C. difficile because the active cytotoxic intermediate formed under anaerobic conditions is:
- A Reduced nitro free radical causing DNA strand breakage ✓
- B Nitroso radical causing protein denaturation
- C Hydroxylamine metabolite inhibiting cell wall synthesis
- D Nitroxide radical inhibiting topoisomerase II
Correct answer: A. Reduced nitro free radical causing DNA strand breakage
Explanation
Under low redox potential in anaerobes, metronidazole's nitro group is reduced by ferredoxin to form cytotoxic nitro free radicals that cause DNA strand breakage and cell death. Option B incorrectly describes the radical type. Options C and D describe incorrect mechanisms and metabolites not associated with metronidazole.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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