A 9 year-old boy presents to your primary care office complaining of scrotal swelling and pain for 3 days. He also has a painful purpuric rash on his lower extremities and buttocks for the last 4 days. The lesions appeared suddenly after a few days of abdominal pain. A CT abdomen did not reveal any abnormality. Abdominal pain is worse after eating. Review of systems revealed joint pains in both knees. He has been taking ibuprofen for the pains, which is helping him. Ten days ago he had been diagnosed with a case of pharyngitis and was given a course of antibiotics. He denies nausea, vomiting, fever and gross hematuria. Physical examination is significant for palpable purpura over the lower extremities and buttocks. Laboratory tests reveal blood and protein in his urine and fecal occult blood test is positive. Of the following, the most likely diagnosis in this patient is
