A 40-year-old woman with a 5-year history of arthritis, photosensitivity, alopecia, and urinary protein (+). For the past month, she has had pitting edema of both lower limbs, oliguria, BP 168/100 mmHg. Lab findings: urinary protein (++), red blood cells 10-15/HPF, Hb 100g/L, Scr 230 μmol/L, ANA 1:640 (membranous pattern), anti-Smith antibody positive, serum C3 0.5 g/L. The most likely diagnosis is: