A 30-year-old female presents with facial rash, bilateral lower extremity edema for 2 years, and decreased urine output for 1 week. BP 150/90 mmHg, urinary protein 6.5g/d, RBCs in full field of view, serum ALB 26g/L, serum creatinine 256μmol/L, C<sub>3</sub> decreased, ANA(+), and ultrasound shows slightly enlarged kidneys. The most likely diagnosis is: