A 28-year-old woman from rural Rajasthan presents with bone pain, muscle weakness, and recurrent fractures. On examination, she has bowing of legs and frontal bossing. Serum calcium is 7.2 mg/dL, phosphate 2.8 mg/dL, alkaline phosphatase 180 IU/L, and 25-hydroxyvitamin D is 12 ng/mL. X-rays show loss of sharp corticomedullary differentiation and Looser zones. What is the most appropriate immediate next step in management?