A 38-year-old woman from rural Bihar presents with progressive bone pain, muscle weakness, and difficulty climbing stairs for the past 8 months. She reports minimal sun exposure due to cultural practices. On examination, she has muscle tenderness and a positive Trendelenburg sign. Laboratory investigations reveal: serum calcium 7.2 mg/dL, phosphate 2.8 mg/dL, alkaline phosphatase 156 IU/L, 25-OH vitamin D 12 ng/mL (normal >30 ng/mL), and PTH 145 pg/mL (normal 15–65 pg/mL). X-ray of the femur shows loss of sharp corticomedullary differentiation and Looser zones. What is the most likely diagnosis?