A previously healthy 16-year-old girl is admitted to the hospital for evaluation of fever and abdominal pain. The fever (as high as 104°F [40°C]) began 5 days before admission. She has concurrent nausea, vomiting, abdominal pain, and diarrhea. There is no blood or bile in the vomitus, and stools are without blood or mucus. She has no headache or rash. She has no known sick contacts and is a recent immigrant from Pakistan. Her medical history and family history are unremarkable.

On admission, she appears unwell, but nontoxic. Her vital signs are notable for temperature of 102.6°F (39.2°C), pulse of 92 beats/min, and blood pressure of 111/71 mm Hg. Findings from physical examination are normal except for a tender abdomen without masses or organomegaly.

Initial laboratory evaluation is significant for a low white blood cell count of 3,100/μL (3.1×109/L), a low hemoglobin level of 10.1 g/dL (101 g/L),...

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