A 38-year-old woman with systemic lupus erythematosus (SLE) develops acute lupus nephritis (Class IV) with rapidly declining renal function (creatinine rising from 0.9 to 3.2 mg/dL over 2 weeks). Kidney biopsy shows proliferative glomerulonephritis with active endocapillary proliferation and wire-loop lesions, indicating reversible glomerular injury. Which drug is the preferred first-line induction therapy to prevent progression to irreversible renal fibrosis and end-stage renal disease?