A 16-year-old girl presents to the ED with persistent fever and headache for 9 days. She was previously healthy except for a tooth extraction 3 weeks ago, for which she received penicillin. She denies shortness of breath, visual changes, vomiting, diarrhea, palpitations, joint pain, weight loss, cough, or urinary symptoms. She had been seen repeatedly in the ED and had normal examination findings and unremarkable CBC, CSF analysis, and urinalysis. She was given intramuscular ceftriaxone and sent home with a prescription for oral azithromycin. After returning to the ED today, she is admitted.

Physical examination shows a temperature of 102.2°F (39.0°C), heart rate of 120 beats/min, respiratory rate of 18 breaths/min, and blood pressure of 103/71 mm Hg. Auscultation of the heart reveals a mid-systolic click followed by a 2/6 crescendo-decrescendo murmur.

Her WBC count is 11.0×103/mcL (11.0×109/L) with 27% bands. Chest radiography demonstrates normal findings....

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