A 35-year-old woman with systemic lupus erythematosus presents with new-onset facial puffiness and significant leg swelling. Laboratory tests show a serum albumin of 1.8 g/dL and a urine protein-to-creatinine ratio of 5.0 g/g. A renal biopsy confirms membranous lupus nephritis. Question: Which of the following is the most appropriate initial treatment for this patient?