Patient: 7-year-old male Presenting Complaint: Rash, abdominal pain, and joint pain for 3 days History of Present Illness: The child’s mother reports that he developed a red rash on his legs and buttocks 3 days ago. He also complains of colicky abdominal pain and knee pain. The rash initially started as small red spots but has become more raised and purplish. He has had no fever but feels tired. The abdominal pain is intermittent and non-radiating, and he has had one episode of dark-colored stool. No history of recent trauma, but he had an upper respiratory infection (sore throat and runny nose) 1 week ago. No known drug allergies or recent vaccinations. Past Medical History: No known chronic illnesses; Fully vaccinated Family History: No history of autoimmune diseases or kidney problems Physical Examination: Vital signs: BP 100/70 mmHg, HR 95 bpm, RR 18 bpm, Temp 36.8°C Skin: Palpable, non-blanching purpura on the lower extremities, buttocks, and thighs Joints: Mild swelling and tenderness in both knees with limited range of motion due to pain, but no joint effusion Abdomen: Mild tenderness in the periumbilical region, no rebound tenderness or guarding Lungs and heart: Normal Laboratory Results: Complete blood count: Normal platelet count, mild leukocytosis Serum IgA: Elevated Urinalysis: Hematuria (+2), mild proteinuria Stool occult blood test: Positive Which of the following is the most likely diagnosis?
