A 6-year-old (20 kg) receives a first dose of IV ampicillin in the ED. Within 5 minutes the child develops urticaria, lip and tongue swelling, audible wheezing, BP 70/40, HR 150. Which is the first priority intervention?

This is anaphylaxis — the IgE-mediated, life-threatening hypersensitivity that follows minutes after an antigen exposure with multisystem features (skin, respiratory, cardiovascular, GI). The first-line drug is intramuscular epinephrine; delay and use of antihistamines or steroids alone are common errors that increase mortality. Pediatric dose: 0.01 mg/kg of 1 mg/mL (1:1000) IM into the anterolateral thigh, max single dose 0.3 mg pre-pubertal or 0.5 mg adult, repeat every 5 to 15 minutes as needed. Concurrent actions: stop the offending antibiotic, call rapid response or code, support airway with oxygen and prepare for intubation if airway compromise progresses, place supine with legs elevated unless respiratory distress favors upright, IV access for normal saline bolus 20 mL/kg as needed for hypotension; nebulized albuterol for bronchospasm; H1 (diphenhydramine) and H2 (famotidine) antihistamines and corticosteroid (methylprednisolone) as adjuncts only after epinephrine. Monitor for biphasic reaction up to 8 to 12 hours. Document, prescribe an epinephrine auto-injector at discharge with teaching, and arrange allergy follow-up. Diphenhydramine alone, slowing the antibiotic, or fluid alone are unsafe and miss the priority of epinephrine.

Anaphylaxis = IM epinephrine first. Antihistamines and steroids are adjuncts, not substitutes. Anterolateral thigh is the preferred site.