A 7-year-old girl is brought to the emergency department because she has had abdominal pain and vomiting for the past day. The pain is intermittent, diffuse in nature, and worse after meals. She does not have loose or bloody stools. She has had a nonpruritic rash on her legs and swelling of her ankles for 6 days. She was treated with oral amoxicillin for a sore throat 2 weeks ago. Her immunizations are up-to-date. Vital signs are within normal limits. Examination of the lower extremities shows non-blanching, raised erythematous papules. The left ankle joint is swollen and warm, and its range of motion is limited by pain. Abdominal examination shows tenderness to palpation in the left lower quadrant without guarding or rebound. Bowel sounds are normal. Test of the stool for occult blood is positive. Laboratory studies show:Hemoglobin 10.1 g/dLLeukocyte count11,800/mm3Platelet count 431,000/mm3Erythrocyte sedimentation rate 45 mm/hSerum Glucose 72 mg/dLCreatinine 0.9 mg/dLWhich of the following is the most likely diagnosis?

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