**Core Concept**
The metacarpal index is a measure of the length of the metacarpals in relation to the width of the hand, which is used to assess skeletal abnormalities associated with various conditions, including acromegaly, hypothyroidism, and Turner syndrome.

**Why the Correct Answer is Right**
The normal metacarpal index is typically calculated as the ratio of the length of the 2nd metacarpal bone to the width of the hand, usually expressed in millimeters. An index of 8-10 mm is considered normal. This measurement is an important diagnostic tool for identifying individuals with skeletal abnormalities that may be indicative of certain endocrine disorders or genetic conditions.

**Why Each Wrong Option is Incorrect**
* **Option A:** A metacarpal index of 6 mm is considered low, which may be indicative of conditions such as hypothyroidism or Turner syndrome, so this is incorrect.
* **Option B:** A metacarpal index of 12 mm is considered high, which may be indicative of conditions such as acromegaly, so this is incorrect.
* **Option C:** A metacarpal index of 5 mm is considered very low, which may be indicative of severe skeletal abnormalities, so this is incorrect.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the metacarpal index is a useful diagnostic tool for identifying skeletal abnormalities associated with various conditions, and a normal index can help rule out certain endocrine disorders or genetic conditions.

**Correct Answer: C. 9 mm**