A 3-year-old girl is brought to the emergency department because of high fever for 3 days, pharyngitis,and an erythematous, pruritic rash. The rash appeared 2 days ago, and a physician had prescribed cephalexin. There is no history of allergy, recent travel, or contact with ill people.

On physical examination, the girl does not appear toxic. She has a rectal temperature of 38.4°C(101.2°F), pulse of 180 beats/min,respiratory rate of 34 breaths/min,and blood pressure of 100/67 mm Hg. There is a generalized erythematous rash as well as urticarial lesions. No evidence of skin infection is found. The conjunctivae are injected, and her lips are red and dry. The throat is diffusely reddened. Her lymph nodes are not enlarged. No heart murmurs are heard, and the lungs are clear. Findings on the remainder of the physical examination are normal.

Laboratory results include: white blood cell count, 14.4 × 109/L(14.4 × 10...

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