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?

**Core Concept**
The child's presentation, including blue-tinged sclera, hearing loss, and small, slightly blue, misshapen teeth, along with numerous fractures, suggests a condition involving abnormal metabolism of a specific substance. This condition is associated with a genetic disorder that affects multiple systems, including the skeletal, auditory, and ocular systems.

**Why the Correct Answer is Right**
The child's symptoms are characteristic of a disorder involving abnormal metabolism of vitamin C (ascorbic acid). Vitamin C is essential for the synthesis of collagen, a protein crucial for bone, skin, and connective tissue integrity. Deficiency in vitamin C leads to impaired collagen synthesis, resulting in fragile bones (osteoporosis), blue-tinged sclera (due to poor collagen synthesis in connective tissue), and other systemic manifestations. The presence of numerous fractures without significant bruising suggests that the child's bones are prone to fractures due to their fragile nature.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not relate to the child's symptoms. Vitamin A is essential for vision, immune function, and reproduction, but its deficiency does not cause the symptoms described.

**Option B:** This option is incorrect because it is unrelated to the child's condition. Vitamin K is necessary for blood clotting, but its deficiency does not cause the symptoms of fragile bones, hearing loss, or blue-tinged sclera.

**Option C:** This option is incorrect because it is not directly related to the child's symptoms. Vitamin E is an antioxidant essential for skin health and immune function, but its deficiency does not cause the symptoms described.

**Option D:** This option is incorrect because it is not the correct substance involved in the child's condition. Vitamin D is essential for bone health, but its deficiency leads to rickets, a condition characterized by softening of the bones, not fragile bones prone to fractures.

**Clinical Pearl / High-Yield Fact**
This condition, known as scurvy, is a classic example of a genetic disorder caused by a deficiency of a specific vitamin. It highlights the importance of vitamin C in maintaining collagen synthesis and preventing fragile bones.

**Correct Answer:** C.

✓ Correct Answer: A. Collagen