A 30-year-old woman presents with lower extremity edema for 2 weeks. Physical examination: blood pressure (BP) 200/100 mmHg, urinary protein (+++), red blood cells (RBC) 10-15/HPF, serum creatinine (Cr) 150 μmol/L, serum albumin (Alb) 32 g/L. The next diagnostic step for this case should be: