A family suspected of child abuse because one of their children has had multiple fractures consults an ohopedic specialist at a children's hospital. The specialist examines the child, who is now 5 years old, and notes that the child has blue-tinged sclera, hearing loss, and small, slightly blue, misshapen teeth. Radiologic studies confirm the presence of numerous fractures of various ages. No significant degree of bruising is seen over sites of recent fracture. The disease this child most likely has is related to abnormal metabolism involving which of the following substances?