A 22-month-old exclusively breastfed toddler from northern India presents with delayed walking, bowlegs, widened wrists, rachitic rosary, and delayed fontanelle closure. Serum 25-hydroxyvitamin D is 6 ng/mL (severely deficient), PTH is 180 pg/mL (elevated), and alkaline phosphatase is 850 U/L (markedly elevated). Wrist radiograph demonstrates the findings marked **A** in the diagram. Which of the following best explains the radiographic appearance at this site?