A 3-year-old girl is brought by her mother for evaluation of a rash that began as a single painful, itchy blister on the lip 4 days earlier. After the blister resolved, similar lesions appeared on the face, trunk, and upper extremities. The mother reports that the child had cold symptoms a week before presentation, but no fever, chills, body aches, nausea, or vomiting. The patient has a history of atopic dermatitis and both parents have asthma. She has had no hospitalizations or surgeries and no recent medication use. She is up-to-date with her immunizations.