Osteomalacia

A 62-year-old woman presents with diffuse bone pain, proximal muscle weakness, and waddling gait for one year. She has limited sun exposure, history of partial gastrectomy, and vegetarian diet low in dairy. Laboratory studies show 25-hydroxyvitamin D 8 ng/mL (severely deficient), serum calcium 8.2 mg/dL (low-normal), phosphate 2.0 mg/dL (low), and alkaline phosphatase 425 U/L (markedly elevated). Pelvic radiograph demonstrates symmetric bilateral transverse lucent lines perpendicular to the cortical bone in the femoral necks and pubic rami, as marked in **B** in the diagram. These represent Looser's pseudofractures. Which of the following best describes the pathophysiologic mechanism underlying the radiographic findings marked **B**?