A 58-year-old man with long-standing hypertension and type 2 diabetes presents with bilateral ankle oedema and frothy urine. Blood pressure is 150/95 mmHg. Urinalysis shows 3+ protein. Laboratory tests show albumin 25 g/L and creatinine 115 µmol/L (eGFR 60 mL/min/1.73 m2). What is the most appropriate additional investigation to quantify renal involvement?