A previously healthy 9-year-old boy presents with 2 days of fever, nonbloody and nonbilious emesis, abdominal pain, headaches, and generalized myalgia. The patient recently fished in several creeks and lakes in Ohio and owns a pet dog, but he denies water ingestion, tick bites, contact with farm animals or reptiles, or travel out of state. He has no known coronavirus disease 2019 (COVID-19) exposures. His brother was recently ill with similar symptoms but has since recovered. There is a remote family history of lupus and celiac disease.

Physical examination reveals a well-appearing child who is febrile to 103.1°F (39.5°C), with a heart rate of 98 beats/min, respiratory rate of 26 breaths/min, and blood pressure of 115/69 mm Hg. He is tender in the right upper quadrant without guarding, rebound, or hepatosplenomegaly. He also has mild tenderness of his quadriceps muscles, but no extremity swelling or joint pain. His skin examination...

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