A 9-year-old boy with recently diagnosed IgA vasculitis is being followed up for persistent hematuria and proteinuria. He initially presented with palpable purpura, knee pain, and abdominal pain that resolved after 2 weeks of supportive care. His urine dipstick test continues to show persistent proteinuria, and his serum creatinine level is mildly elevated. Which of the following is the most appropriate next step in management?
