Henoch-Schönlein Purpura Palpable Purpura

A 7-year-old boy presents with a 2-week history of symmetric palpable purpura on the lower extremities and buttocks following a recent upper respiratory infection. He also complains of colicky abdominal pain and ankle arthralgia. On examination, the lesions marked **A** in the diagram are non-blanching and do not blanch with pressure. His platelet count is 250,000/µL (normal). Based on the clinical presentation and the characteristic appearance of the lesions marked **A**, what is the most likely diagnosis?