A 10-year-old boy presents to the pediatric clinic with a history of palpable purpuric rash on his lower extremities, mild abdominal pain, and joint aches over the past week. His parents report that his symptoms began shortly after a recent upper respiratory infection. Laboratory studies reveal the presence of microscopic hematuria on urinalysis. Based on his clinical presentation, he is diagnosed with Henoch-Schönlein Purpura (IgA vasculitis). In patients diagnosed with Henoch-Schönlein Purpura, which of the following represents the most frequent serious complication that typically occurs within the first month of illness?
