A 72-year-old man with a 10-year history of chronic kidney disease (eGFR 35 mL/min/1.73 m²) presents with progressive bone pain and muscle weakness. Biochemical investigations reveal: serum calcium 7.2 mg/dL, phosphate 5.8 mg/dL, alkaline phosphatase 95 U/L, and intact PTH 680 pg/mL (normal 15–65). X-ray of the pelvis shows a 'rugger jersey spine' appearance. DEXA scan T-score is −2.8. Which of the following best describes the underlying bone pathology in this patient?