**Core Concept**
The medullary index is a ratio used to assess the proportion of medullary cavity to the bone's total diameter. It is an essential parameter in forensic medicine, anthropology, and orthopedics. The reliability of bones for calculating the medullary index depends on their cortical thickness and medullary cavity size.

**Why the Correct Answer is Right**
The medullary index is most reliably calculated in bones with a relatively uniform cortical thickness and a large medullary cavity. Long bones like the femur and humerus are ideal candidates due to their cylindrical shape and consistent cortical thickness. In contrast, bones with a complex shape or varying cortical thickness, such as the pelvis or vertebrae, are less reliable for medullary index calculations.

**Why Each Wrong Option is Incorrect**
**Option A:** Femur - Incorrect because the femur is actually one of the most reliable bones for medullary index calculations due to its uniform cortical thickness and large medullary cavity.
**Option B:** Humerus - Incorrect because the humerus, like the femur, has a relatively uniform cortical thickness and a large medullary cavity, making it a reliable bone for medullary index calculations.
**Option C:** Pelvis - Incorrect because the pelvis has a complex shape and varying cortical thickness, making it less reliable for medullary index calculations.

**Clinical Pearl / High-Yield Fact**
When assessing the reliability of bones for medullary index calculations, remember that long bones with a cylindrical shape and uniform cortical thickness are generally more reliable than bones with complex shapes or varying cortical thickness.

**Correct Answer: C. Pelvis.**