Henoch-Schönlein Purpura (IgA Vasculitis)

A 6-year-old boy presents with a 2-week history of symmetric, non-blanching purpura over his lower limbs and buttocks following a sore throat. On examination, he has palpable purpura, mild ankle swelling, and colicky abdominal pain. Platelet count is 250,000/μL. The condition marked **C** in the diagram is suspected. Which of the following is the MOST IMPORTANT long-term complication that requires surveillance in this child?

A 7-year-old boy presents two weeks after a sore throat with a one-week history of crops of non-blanching purpura on the buttocks and extensor surfaces of the lower limbs. He also complains of colicky periumbilical abdominal pain and swelling of both ankles and knees. Urinalysis shows microscopic hematuria. Platelet count is normal (245,000/μL). The clinical presentation shown in structure **B** is most consistent with which of the following pathophysiological mechanisms?