**Core Concept**
Keratin is a type of structural protein found in the skin, hair, and nails of humans and many other animals. It is a key component of the cytoskeleton and provides mechanical strength and rigidity to these tissues.

**Why the Correct Answer is Right**
Keratin is a type of intermediate filament protein that is composed of a central alpha-helical coiled-coil domain surrounded by a non-helical terminal domain. This structure allows keratin to form strong, flexible filaments that can withstand mechanical stress. In the skin, keratin is produced by keratinocytes and is an essential component of the epidermal barrier, providing protection against water loss and external damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as keratin is not a type of glycoprotein.
**Option B:** Incorrect, as keratin is not a type of enzyme involved in protein synthesis.
**Option C:** Incorrect, as keratin is not a type of collagen found in connective tissue.

**Clinical Pearl / High-Yield Fact**
Keratin is a key component of the skin's natural barrier function, and defects in keratin expression can lead to conditions such as epidermolysis bullosa simplex, a rare genetic disorder characterized by skin fragility and blistering.

**Correct Answer: C. Keratin is a type of structural protein found in the skin, hair, and nails.**