A 30-year-old female patient with a 12-year history of proteinuria and intermittent hematuria developed gross hematuria 2 days after a cold. BP 146/94 mmHg, urinary protein (++++)(5g/d), urine sediment shows full-field dysmorphic RBCs, normal renal function, serum ALb 20g/L, Hb 70g/L, ANA 1:80, complement C3 0.5g/L. The most likely diagnosis is: