For a patient with a severe allergic reaction to penicillin, which β-lactam is the best choice for treating a urinary tract infection?

In the context of a patient with a severe allergic reaction to penicillin, the most appropriate choice is aztreonam. Aztreonam is a monobactam antibiotic that has a β-lactam structure but lacks cross-reactivity with penicillin. This makes it a suitable alternative for individuals who have a documented allergy to penicillin, allowing for safe treatment of infections such as urinary tract infections without the risk of an allergic reaction.

Cefepime, ertapenem, and ceftaroline are all classified as cephalosporins or carbapenems, which may present a risk of cross-reactivity in patients with penicillin allergies, especially if the reaction was severe. Therefore, these options would not be advisable for a patient with a history of severe allergic reactions to penicillin.