A 28-year-old male presents with edema and oliguria for 1 week, with a history of 'cold' 2 weeks ago. BP: 150/90 mmHg, Hb 80 g/L, urinary protein (+++), occult blood (+++), urinary RBCs 15-20/HP, serum creatinine 384 μmol/L. Ultrasound shows left kidney 11.1×4.1×5.2 cm, right kidney 12.2×4.8×5.4 cm. What is the next best step?