A 9-year-old boy is brought to the emergency department with a 4-day history of fever, abdominal pain, and a rash on his lower legs. On examination, temperature is 38.5˚C (101.3˚F), pulse is 110/min, respirations are 20/min, and blood pressure is 90/60 mm Hg. Physical examination reveals a petechial rash on the lower legs and ankles. Abdominal examination is notable for tenderness in the right lower quadrant. Urinalysis shows mild hematuria and proteinuria. Laboratory tests show hemoglobin 12 g/dL (11.5-14.5), leukocytes 6200/uL (4500-13000), platelets 246000/uL (187000-400000), creatinine 0.5 mg/dL (0.3-0-7), blood urea nitrogen 12 mg/dL (6-24). Coagulation studies were normal. Which of the following is the most likely diagnosis?