A 6-year-old girl presents with a 2-week history of a skin rash over her buttocks and legs and joint pain. The parents report seeing blood in the urine. Physical examination reveals palpable purpuric skin lesions and markedly swollen knees. The results of laboratory studies reveal abnormally high erythrocyte sedimentation rate (30 mm/h), BUN of 25 mg/dL, and serum creatinine of 3 mg/dL. Urinalysis demonstrates RBCs and RBC casts. The stool guaiac test is positive. Biopsy of lesional skin reveals deposits of IgA in the walls of small blood vessels. Which of the following is the most likely diagnosis?