A patient allergic to penicillin was prescribed cephalexin and developed shortness of breath 30 minutes later. Which molecule is primarily responsible for this reaction?

Correct answer: C. Histamine

• A. INCORRECT — Serotonin plays a minor role in allergic responses and is not the primary mediator of penicillin/cephalosporin cross-reactivity anaphylaxis.
• B. INCORRECT — Bradykinin is more relevant to ACE inhibitor-induced angioedema, not beta-lactam anaphylaxis.
• C. CORRECT — This describes an IgE-mediated anaphylactic reaction due to cross-reactivity between penicillins and cephalosporins (sharing a beta-lactam ring). Mast cells and basophils release HISTAMINE as the primary mediator, causing bronchoconstriction, vasodilation, urticaria, and angioedema.
• D. INCORRECT — Epinephrine is the TREATMENT for anaphylaxis, not a mediator released. It counteracts histamine's effects.