Osteopetrosis Marble Bone

An 8-year-old boy presents with recurrent pathological fractures from minor trauma, progressive vision loss, hearing impairment, and facial palsy. Laboratory studies show pancytopenia (Hb 6.5 g/dL, platelets 45,000/μL, WBC 3,200/μL). Skeletal survey demonstrates the radiological findings marked as **A** in the diagram—diffuse sclerosis with bone-in-bone appearance, sandwich vertebrae, and Erlenmeyer flask deformity of the femur. Despite markedly increased bone density, the bones are paradoxically brittle and prone to fractures. Which of the following best explains the pathophysiology underlying this clinical presentation?