A previously healthy 13-year-old girl presents with 7 days of fever, congestion, and cough. Five days before presentation she was seen by her primary care physician and diagnosed clinically as having community-acquired pneumonia and started on amoxicillin. Because of continued fevers, her primary care physician changed her antibiotic to cefdinir after 3 days. She presented to the emergency department 2 days later with dyspnea and worsening cough. She had no other preceding illness or sick contacts.

On presentation she is afebrile and vital signs include a heart rate of 136 beats/min, a respiratory rate of 56 breaths/min, and oxygen saturation of 87% on room air. Her growth chart demonstrates weight and length below the third percentile for age with no recent weight loss. The physical examination is significant for a thin female with pallor, dry mucous membranes, pale conjunctivae, dyspnea with bilateral decreased breath sounds and diffuse crackles, and nonblanching,...

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