Question

Which one of the following antibiotics is NOT effective against Pseudomonas aeruginosa?

• A. Ceftazidime
• B. Piperacillin
• C. Vancomycin
• D. Aztreonam

Hint:

Think about which of these four belongs to a drug class that only works on gram-positive bacteria.

Answer 1

The Correct Option is C

This question tests recognition of which antibiotic classes actually reach gram-negative targets like Pseudomonas.

1. Ceftazidime: a cephalosporin built for anti-pseudomonal cover, so it is effective and not the answer.
2. Piperacillin: an extended-spectrum penicillin with established Pseudomonas activity, so it is effective too.
3. Vancomycin: a glycopeptide restricted to gram-positive organisms because it cannot cross the gram-negative outer membrane — it has no activity against Pseudomonas, making it the correct answer.
4. Aztreonam: a monobactam active specifically against gram-negative aerobes including Pseudomonas, so it is effective and not the answer.

Correct answer: Vancomycin, since it works only on gram-positive bacteria and cannot penetrate Pseudomonas.