A 10-year-old girl presents with a 3-day history of colicky abdominal pain and dark-colored urine. Her parents report that she had non-blanching purple spots on her legs a week ago that have now faded. On examination, she has no current rash but mild periumbilical tenderness. Urinalysis reveals hematuria and proteinuria. Which of the following tests is most appropriate to confirm the diagnosis of IgA vasculitis (Henoch-Schönlein purpura) if uncertainty remains?
