A four-year-old male is brought in with a rash and leg pain. The child is irritable with normal vital signs. The child has clear nasal discharge and an occasional cough from an upper respiratory infection that is resolving. The legs have palpable purpura, and his knees are swollen and painful with range of motion. Urinalysis shows 2+ hematuria, CBC is normal, and coagulation studies are normal. What is the most likely diagnosis?