A 28-year-old woman with biopsy-proven lupus nephritis (Class IV, diffuse proliferative) presents with proteinuria 3.5 g/day, serum creatinine 1.8 mg/dL, and active urinary sediment. She is currently on prednisolone 1 mg/kg/day. Complement levels (C3, C4) are low, and anti-dsDNA titres are elevated. What is the most appropriate next step in management?