A 30-year-old female developed fever and arthralgia after catching a cold, lasting 2 months. She was hospitalized with generalized edema and oliguria for 1 week. X-ray examination revealed pericardial and pleural effusion. Laboratory tests showed: urinary protein (+++), plasma albumin 24g/L, hemoglobin 83g/L, reticulocyte count 0.20 (20%), platelets 60×10⁹/L, anti-dsDNA antibody (+), and erythrocyte sedimentation rate 110mm/1h. What is the first-line treatment for this patient?