A 62-year-old woman with CKD stage 4 (eGFR 18 mL/min/1.73m²) secondary to chronic glomerulonephritis presents for routine follow-up. She is asymptomatic, hemoglobin is 9.2 g/dL, serum phosphate is 5.8 mg/dL, serum calcium is 8.1 mg/dL, and intact PTH is 285 pg/mL (normal 15–65 pg/mL). She is not yet on any renal replacement therapy. What is the most appropriate next step in management?