Clindamycin is effective against all of the following EXCEPT:

Correct answer: C. Pseudomonas aeruginosa

• A. Clindamycin has excellent anaerobic gram-negative coverage, including Bacteroides and Prevotella.
• B. Anaerobic gram-positive cocci are well covered by clindamycin.
• C. Correct. Clindamycin has no meaningful activity against aerobic gram-negative rods. Pseudomonas is intrinsically resistant — it requires antipseudomonal agents such as piperacillin-tazobactam or ciprofloxacin.
• D. Clindamycin covers gram-positive cocci including viridans streptococci.

Note: Clindamycin's most important adverse effect is Clostridioides difficile-associated pseudomembranous colitis.