A 13-year-old previously healthy immunized boy presents to the pediatric emergency department (ED) with fever, hives, and red-colored urine. He had no history of anaphylaxis, but earlier that morning he had an episode of angioedema, hives, and difficulty breathing. His mother treated him with 25 mg of oral diphenhydramine and 0.3 mg of intramuscular epinephrine obtained from a sibling with a history of anaphylaxis. All of his symptoms improved by the time emergency medical services arrived so he was not transported to a hospital. A couple of hours later he presented to his pediatrician due to recurrence of hives. He was prescribed hydroxyzine and a prednisolone course for presumptive allergic reaction symptoms. That night (12 hours after intramuscular epinephrine administration), his hives were still visible but much improved according to his mother. However, he became febrile to 101.9°F (38.8°C) and also noticed red-colored urine, so he presented to the ED....

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