A 12-year-old boy is brought to the office for evaluation of hives. He has no significant past medical history and no history of allergies. He has just joined the middle school soccer team and noticed that he gets hives about 10 minutes into practice. The hives are itchy and consist of irregular blotches on his legs and trunk, about 10–20 cm in size, and they persist for about 30 minutes. He does not experience swelling of the lips or oropharynx and denies any wheezing or shortness of breath. ...