A 5-year-old boy presents with nephrotic syndrome (proteinuria 3.5 g/day, hypoalbuminemia, edema). Serum creatinine and complement levels are normal. Light microscopy of renal biopsy shows no glomerular abnormality. Immunofluorescence is negative. Electron microscopy reveals diffuse flattening of the structures marked **C** in the diagram. Which of the following is the most appropriate next step in management?