A 9-year-old Hispanic boy is brought to the emergency department because of fever, headache, muscle aches, and abdominal pain for 2 days. He says that he has had no cough, vomiting, diarrhea, constipation,or urinary symptoms. There is no history of skin rash, trauma, or distant traveling. The other family members have been healthy. Two weeks prior to this episode the patient had been evaluated for fever and severe abdominal pain. He was observed for 8 hours, acute appendicitis was ruled out, and his symptoms resolved within 48 hours.

Physical examination reveals a temperature of 38.8°C (102°F), heart rate of 146 beats/min,diffuse abdominal tenderness without signs of peritoneal irritation, pain on palpation of the calves of both legs, and pain in the right ankle with passive movement (but no erythema, heat, or swelling). His pharynx is free of erythema and exudate. There are no skin lesions. The neurologic examination reveals generalized...

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