A 56-year-old male presents with lumbosacral pain for over 1 year and proteinuria for 3 months. Investigations: erythrocyte sedimentation rate (ESR) 23mm/h, hemoglobin (Hb) 8g/dl, urinary protein electrophoresis shows predominantly low molecular weight proteins, serum creatinine (Scr) 152μmol/L. What is the preferred diagnostic test to identify the etiology?