A 32-year-old woman with biopsy-proven SLE (Class III lupus nephritis) has completed 6 months of induction therapy with mycophenolate mofetil (MMF) 3 g/day and prednisolone 0.5 mg/kg/day. Repeat urinalysis shows persistent proteinuria (1.5 g/24 h) and active urinary sediment. Serum creatinine is stable at 1.2 mg/dL. What is the most appropriate next step?