A 24-year-old female presents with high fever and arthralgia for half a month, edema for 10 days, and blood pressure (BP) of 150/90 mmHg. Over the past 7 days, she has developed apathy and hypophonia. For the last 3 days, her urine output has been 200–400 mL/day, with urinary protein (+++), urinary red blood cells (RBCs) 15–20/HP. Laboratory tests show hemoglobin (Hb) 80 g/L, C3 0.42 g/L, albumin (Alb) 23 g/L, and serum creatinine (Scr) 400 μmol/L. The most likely diagnosis is: