A 34-year-old woman presents with persistent microscopic hematuria and intermittent urinary discomfort for 3 years. Hb 120g/L, urinary protein 2g/24h, urinary RBCs 20-30/HP, WBC 2-3/HP. Urinary RBC phase-contrast microscopy shows: 20% normal morphology, 80% dysmorphic. The most critical next step is: