How would you approach a patient without a history of chronic urticaria who presents with urticaria to multiple antibiotics of different classes (penicillin, cephalosporins, TMP/sulfa, azithromycin)?

While theoretically you could formally evaluate (and try to delabel) all 4 classes of antibiotics, generally it makes sense to focus on the ones that are clinically most useful - which are 1) the beta-lactams, 2) macrolides, and 3) sulfonamides. For a typical patient with a history of only urticaria...