Which agent is a monobactam active against Pseudomonas and safe in patients with penicillin allergy?

• A. Aztreonam.
• B. Cefepime.
• C. Imipenem.
• D. Piperacillin.

Answer: (A)

Monobactams offer a beta-lactam option that targets gram-negative rods, including Pseudomonas, with minimal risk of cross-reactivity in patients who are allergic to penicillin. Aztreonam is the only clinically used monobactam, and it inhibits cell wall synthesis by binding bacterial PBPs, giving strong activity against Pseudomonas and other gram-negative aerobes while having little to no activity against gram-positive cocci and anaerobes. Because of its distinct structure, it has a low likelihood of triggering penicillin allergies, making it the preferred choice when a pseudomonal infection is suspected in a penicillin-allergic patient. The other agents belong to different beta-lactam classes or, in the case of penicillins, are not monobactams, so they don’t fit the described criteria.