A mother brings her 5-year-old boy in to your office because she is concerned about a rash on his legs that seems to be worsening. She tells you that he had a runny nose and a mild cough a week earlier, but that those symptoms resolved before the rash developed. He has also complained of "belly pain." The boy's mother says he has been less active and more irritable since the onset of the rash, and that he is hardly eating. She also tells you that earlier in the day, her son told her that it hurts to walk. A complete review of systems is otherwise negative. The 5-year-old was born at term without complication. He has met all developmental milestones and his immunizations are up to date. He takes no medications. The boy's vital signs are normal. He has an erythematous maculopapular rash distributed on his legs symmetrically; it is palpable, nontender, and nonblanching. You detect no abnormalities in abdominal, neurologic, or musculoskeletal examinations. A complete blood count (CBC) and basic metabolic panel (BMP) reveal mild leukocytosis with a normal differential. Urinalysis shows moderate blood and trace protein. Laboratory results are otherwise normal. What's your diagnosis?