**Core Concept**
The cephalic index is a numerical value used in physical anthropology and medicine to quantify the shape and proportions of the human skull. It is calculated by dividing the maximum width of the head by its maximum length, and then multiplying by 100 to express the result as a percentage.

**Why the Correct Answer is Right**
The cephalic index is a useful tool in anthropology and medicine for distinguishing between different human populations and for diagnosing certain medical conditions, such as craniosynostosis. The index is used to categorize skulls into different shapes, including dolichocephalic (long and narrow), brachycephalic (short and broad), and mesocephalic (medium length and width). The cephalic index is calculated using precise measurements of the skull, typically taken from the maximum width and length of the head.

**Why Each Wrong Option is Incorrect**
**Option A:** Skull size - The cephalic index is not a measure of overall skull size, but rather a ratio of width to length.
**Option B:** Facial structure - While the cephalic index may provide some information about facial structure, it is not a direct measure of facial proportions.
**Option C:** Brain size - The cephalic index is not a reliable indicator of brain size or volume.

**Clinical Pearl / High-Yield Fact**
The cephalic index is an important tool in forensic anthropology for identifying human remains and distinguishing between different populations.

**Correct Answer: D. Skull shape**