A 20-year-old female presents with dizziness, fatigue, facial edema, polyarthralgia, hair loss, and oral ulcers for 5 months. Previously diagnosed with 'glomerulonephritis' but treatment was ineffective. Anemia worsened after whole blood transfusion. ESR: 120mm/h, WBC: 4.5×10⁹/L, RBC: 2.5×10¹²/L, Hb: 50g/L, PLT: 5.5×10⁹/L, reticulocytes: 10%. Urinalysis: RBC 5-8/HPF, urinary protein (++++), hyaline casts (+), granular casts (+). BP: 113/75 mmHg, 24-hour urine output: 700ml, normal electrolytes. Renal function: BUN 17.5mmol/L, Cr 420μmol/L. Renal biopsy: 10/15 glomeruli show cellular crescents; diffuse mesangial/endothelial proliferation. Immunofluorescence shows widespread immunoglobulin/complement deposits in capillary walls/mesangium. No current infection. Select the best treatment.