A 25-year-old female is admitted with facial rash and bilateral lower extremity edema for 2 years, and decreased urine output for 2 weeks. Physical examination: BP 150/90 mmHg, maculopapular rash in a butterfly distribution on the face, pitting edema in both lower extremities. Laboratory tests: urine sediment microscopy shows red blood cells (RBCs) covering the entire field, urinary protein 6.5 g/d, serum albumin 27.8 g/L, serum creatinine 254 μmol/L. Blood C3 decreased, antinuclear antibody (ANA) positive, ds-DNA positive. Ultrasound: both kidneys slightly enlarged. The most likely diagnosis is: