A 32-year-old female presents with proteinuria and hematuria discovered 3 years ago. Three years ago, a routine urinalysis (UA) showed urinary protein (+) and microscopic RBCs 10-20/HPF, with blood pressure (BP) 125-135/80-90 mmHg and normal renal function. Over the past 3 years, repeated 24-hour urinary protein quantification showed 0.6-1.4g. One month ago, she developed a sore throat and fever after catching a cold, followed by gross hematuria the next day. Oral antibiotics improved the sore throat, but urinalysis still showed protein (++) and RBCs 12/HPF. Repeat renal function tests showed BUN 10.4 mmol/L and Cr 345 μmol/L. The most valuable diagnostic method is: