A 23-year-old female was admitted with recurrent arthralgia for over a year and edema for 1 month. Physical examination: blood pressure (BP) 150/90 mmHg, marked facial and lower extremity edema, knee joint swelling and mild tenderness. Hemoglobin (Hb) 78g/L, white blood cell (WBC) count 3.0×10<sup>9</sup>/L, platelets (PLT) 50×10<sup>9</sup>/L. Urinalysis: proteinuria (++++), red blood cells (RBC) 10–20/HPF, WBC 5–10/HPF, 24-hour urinary protein quantification 5.6g (non-selective proteinuria), alanine aminotransferase (ALT) 50U, albumin to globulin ratio (A/G) 30/40g/L, serum immunoglobulin G (IgG) 22g/L. The most helpful diagnostic test would be: