Review Question - QID 8250

A 14-year-old boy sustained a right forearm fracture after a moderate-speed fall while skateboarding. He was previously treated for fractures of his left femur and right tibia, all from low-energy mechanisms. He has had some mild vertigo since school age, but otherwise has had an unremarkable birth and growth history. His family history is significant for frequent fractures (his mother). On examination, he has slightly blue sclerae, grossly normal hearing, and normal dentition. Chest, abdomen, and pelvis radiographs reveal a mild curvature of the spine (Cobb angle 10 degrees) and suggestion of a slightly large aortic knob. Findings from a nonaccidental trauma workup were normal. This boy's bones are fragile because of a gene mutation affecting what protein?