The anxious parents of a 9-year-old boy bring him to clinic for the evaluation of progressive sores in his mouth for 2 days and a rash that erupted last night. Nine days earlier, he had felt warm and had a cough with wheezing and abdominal pain. At that time, he was taken to an acute care center where he was diagnosed with pneumonia and prescribed amoxicillin. Overnight, his symptoms worsened, and he was admitted to the hospital. The amoxicillin was discontinued and albuterol, azithromycin, and oral steroids were started. He did well and was discharged after 4 days. Shortly afterward, the mother noted a few blisters on his upper lip.