A 58-year-old man presents with a 3-week history of fever, malaise, and progressive dyspnea. On examination, he has palpable purpura over the lower extremities and ankles, along with mild arthralgia of the knees. Urinalysis shows hematuria with RBC casts. Serum creatinine is 1.8 mg/dL. Which investigation is most appropriate to confirm the diagnosis of IgA vasculitis?