A 35-year-old woman with chronic kidney disease stage 4 (eGFR 22 mL/min/1.73 m²) presents with bone pain, muscle weakness, and a serum calcium of 7.8 mg/dL (corrected). Laboratory studies show phosphate 5.2 mg/dL, alkaline phosphatase 420 IU/L, PTH 580 pg/mL (normal 15–65), and 25-hydroxyvitamin D 28 ng/mL. 1,25-dihydroxyvitamin D level is 18 pg/mL (normal 20–60). What is the most appropriate immediate next step in management?