A 68-year-old man with chronic kidney disease stage 3b (eGFR 35 mL/min/1.73m²) is found on routine screening to have serum calcium 7.8 mg/dL (normal 8.5–10.5), phosphate 5.2 mg/dL (normal 2.5–4.5), and PTH 312 pg/mL (normal 15–65). He is asymptomatic. Alkaline phosphatase is 78 U/L. What is the most appropriate next step in management?