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