You see a patient who says, "my mom told me I was allergic to penicillin." You brush it off until he says, "my throat swelled up, I lost consciousness, I had whole-body hives, and I spent a week in the pediatric intensive care unit." Eeek! Now, you had wanted to use a cephalosporin, but I guess you'll have to use aztreonam (the first molecule shown).
Challenge: Unfortunately this is a clinical pearl that you either know or don't know. Aztreonam has a similar side chain to the beta-lactam drug shown in the second picture. If the patient has a specific allergy to this antibiotic, he should not get aztreonam. What is it?
Both images are in the public domain, from Wikipedia.