Which antibiotic is most suitable for a patient with penicillin allergy due to lack of cross-reactivity?

• A. Amoxicillin
• B. Ceftriaxone
• C. Vancomycin
• D. Aztreonam

Answer: (D)

When a patient has a penicillin allergy, the concern is cross-reactivity with other beta-lactams. Aztreonam is a monobactam with a distinct structure that minimizes or essentially eliminates cross-reactivity with penicillin Allergy. That makes it the safest choice among the options for someone who cannot tolerate penicillins due to this cross-reactivity risk.

Note that aztreonam mainly covers Gram-negative bacteria and does not provide good coverage for Gram-positive organisms or anaerobes, so its use is guided by the infection type. The other beta-lactams listed (penicillins and cephalosporins) carry a nonzero, albeit small, risk of cross-reactivity in penicillin-allergic patients, which is why they’re less suitable in this context. Vancomycin, while not a beta-lactam and not cross-reactive, has a different spectrum and is chosen based on the organism and infection type rather than cross-reactivity concerns.