Which antibiotic should be used for an anaerobic mixed infection insensitive to penicillin?

• A. Amoxicillin
• B. Metronidazole
• C. Clindamycin
• D. Cephalexin

Answer: (C)

When facing an anaerobic infection that isn’t responding to penicillin, you want an antibiotic with strong activity against anaerobes and good tissue penetration, including bone and soft tissue. Clindamycin fits this need well because it has excellent activity against anaerobic bacteria commonly involved in dental infections (such as anaerobic streptococci and Bacteroides) and it can cover many penicillin-resistant strains. It also penetrates well into the tissues often involved in oral infections, which helps achieve effective local concentrations.

Amoxicillin would be unreliable here if the organisms are penicillin-insensitive. A cephalosporin like cephalexin generally has limited anaerobic coverage, making it less ideal for these infections. Metronidazole is highly effective against anaerobes but, by itself, does not reliably cover the aerobic bacteria that may be present in a mixed infection, so it may not suffice as a single agent in such cases.

Thus, clindamycin is the best single-drug choice for an anaerobic mixed infection insensitive to penicillin.