A patient with an acute dentoalveolar abscess reports a documented PENICILLIN allergy. Which antibiotic would you be MOST cautious about prescribing?

Correct answer: B. Cephalexin

• A. AZITHROMYCIN is a macrolide, structurally unrelated to the penicillins, and is a recognised alternative.
• B. CORRECT. CEPHALEXIN is a first-generation CEPHALOSPORIN and, like the penicillins, contains a BETA-LACTAM ring. Cross-reactivity is driven largely by similarity of the R1 side chain, and it is highest with first-generation agents — cephalexin shares an identical side chain with amoxicillin. It is therefore the agent requiring the greatest caution, and it is avoided outright in any patient with a history of anaphylaxis, angioedema or another severe immediate-type reaction.
• C. CLINDAMYCIN is a lincosamide with no beta-lactam ring, no cross-reactivity, and excellent activity against oral anaerobes. It is a standard alternative in the penicillin-allergic patient.
• D. METRONIDAZOLE is a nitroimidazole with no cross-reactivity and potent anaerobic activity, though it lacks aerobic coverage and is typically combined with another agent.

Note: ⚑ Worth discussing as a group, because the modern data have shifted. Older texts quote roughly 10% cross-reactivity; contemporary studies put true cross-reactivity below 2% and attribute most of it to side-chain similarity rather than the beta-lactam ring itself. Many current guidelines now permit a structurally dissimilar cephalosporin after a non-severe penicillin reaction. For exam purposes, the cephalosporin is still the answer to a 'which would you avoid' stem.