Metronidazole would be expected to fail against which of the following anaerobic organisms?
- A Bacteroides fragilis
- B Actinomyces israelii ✓
- C Clostridium perfringens
- D Prevotella melaninogenica
Correct answer: B. Actinomyces israelii
Explanation
Actinomyces species are microaerophilic to facultative organisms with active aerobic metabolic pathways, so they cannot reductively activate metronidazole's nitro group efficiently and are uniformly resistant. Treatment of actinomycosis relies on prolonged high-dose penicillin. In contrast, Bacteroides fragilis, Clostridium species and Prevotella are strict anaerobes whose low-redox ferredoxin-linked systems activate the drug, making them among its most susceptible targets.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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