An 8-year-old child has had abdominal pain and dark urine for 10 days. Physical examination shows blotchy purple skin lesions on the trunk and extremities. Urinalysis shows hematuria and proteinuria. Serologic test results are negative for MPO-ANCA (P-ANCA) and PR3-ANCA (C-ANCA). A skin biopsy specimen shows necrotizing vasculitis of small dermal vessels. A renal biopsy specimen shows immune complex deposition in glomeruli, with some IgA-rich immune complexes. Which of the following is the most likely diagnosis?