A 17-month-old term, previously healthy, fully vaccinated girl presents to the emergency department (ED) with a 5-day history of poor oral intake, fever, and lethargy. Four days earlier she presented to an urgent care facility, where results of influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) polymerase chain reaction (PCR) testing were negative. She was discharged with instructions for supportive care. Her symptoms persisted, prompting a second visit where again she was discharged with similar instructions. Symptoms failed to resolve; therefore, her parents brought her to the ED for further evaluation.

Review of systems is negative for rhinorrhea, cough, vomiting, diarrhea, foul-smelling urine, rash, peeling skin, conjunctival injection, and lymphadenopathy. She lives at home with her parents, older siblings, and grandparents. They reside in the Mid-Atlantic United States and deny recent travel. She has not had much outdoor exposure or any known contact with animals. The patient attends indoor...

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