Penicillin or cephalosporin allergy in septic arthritis necessitates an alternative to cover Gram-negative bacilli; which agent is appropriate?

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

Answer: (A)

In septic arthritis, when the patient cannot receive beta-lactams due to penicillin or cephalosporin allergy, you want an antibiotic that provides reliable Gram-negative bacilli coverage without cross-reactivity concerns. Aztreonam fits this need because it is a monobactam with strong activity against Gram-negative organisms (including Pseudomonas) but no activity against Gram-positive bacteria or anaerobes. Importantly, it has minimal cross-reactivity with penicillins and cephalosporins, making it safe for patients with beta-lactam allergies while still delivering the necessary Gram-negative coverage.

The other beta-lactams listed would pose a risk in a beta-lactam–allergic patient: the cephalosporin and penicillin-class drugs are associated with cross-reactivity, and the carbapenem also falls under beta-lactams with potential, albeit lower, cross-reactivity concerns. Therefore, aztreonam is the appropriate choice for covering Gram-negative bacilli in this scenario.