A 30-year-old female patient is admitted with facial rash, bilateral lower extremity edema for 2 years, and decreased urine output for 1 week. BP 150/90 mmHg, urinary protein 6.5 g/d, RBCs covering the field of view, serum ALB 26 g/L, serum creatinine 256 μmol/L, C3 decreased, ANA(+), and ultrasound shows slightly enlarged kidneys. The most likely diagnosis is: