Henoch-Schönlein Purpura — Palpable Purpura Lower Limbs

A 6-year-old boy presents with a 2-week history of upper respiratory tract infection followed by a rash. Examination reveals the finding marked **A** in the diagram — palpable purpura over the extensor surfaces of the lower limbs and buttocks. His platelet count is 250,000/μL. Which of the following is the most likely diagnosis?

A 7-year-old boy presents with a 10-day history of palpable purpura on his lower limbs and buttocks following an upper respiratory tract infection. On examination, the purpura is non-blanching and raised. The structure marked **C** in the diagram shows the characteristic predilection site for the rash in this condition. Which of the following best explains why the purpura in Henoch-Schönlein purpura predominantly affects the **extensor surfaces** of the lower extremities and buttocks rather than the trunk and upper limbs?