Explanation:
The correct option is a. The clinical and radiographic findings described are pathognomonic for Osteopetrosis (Albers-Schönberg disease or 'marble bone disease'). Despite the bones appearing massively thick and radiodense, they are structurally abnormal, highly brittle, and extremely prone to pathologic fractures. The fundamental defect lies entirely within the osteoclasts. Genetic mutations (commonly involving carbonic anhydrase II or the chloride channel CLCN7) prevent the osteoclast from acidifying the sealing zone at the ruffled border. Without an acidic environment, the osteoclast is functionally paralyzed and completely unable to dissolve hydroxyapatite or resorb bone. This arrests the normal bone remodeling process, leading to a failure of medullary canal formation (causing severe extramedullary hematopoiesis and pancytopenia) and a failure to remodel the metaphyses (resulting in the Erlenmeyer flask deformities). Reference: Tolar J et al. Osteopetrosis. N Engl J Med.