A 28-year-old woman from Delhi presents with nephrotic syndrome (proteinuria 8 g/day, serum albumin 2.1 g/dL, edema). Serology is negative (ANA, ANCA, anti-GBM). Renal ultrasound shows normal-sized kidneys with preserved echotexture. She has no systemic features and normal complement levels. What is the most appropriate next step in management?