**Core Concept**
Collagen is a type of structural protein found in the connective tissue of animals, including humans. It provides strength and elasticity to skin, bones, tendons, and ligaments. The unique amino acid composition of collagen is responsible for its exceptional mechanical properties.

**Why the Correct Answer is Right**
Collagen is a polypeptide composed of three chains of amino acids, known as alpha-helices, which are twisted together to form a triple helix structure. The main amino acid responsible for this structure is **hydroxyproline**, which is derived from the amino acid **proline**. Hydroxyproline is essential for the formation of collagen's triple helix structure, which is stabilized by hydrogen bonds between the hydroxyl group of hydroxyproline and the carbonyl group of other amino acids.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glycine is the most abundant amino acid in collagen, but it is not the specific amino acid that gives collagen its unique structure.
* **Option B:** Alanine is a non-essential amino acid found in collagen, but it is not directly responsible for its structural properties.
* **Option C:** Hydroxylysine is another amino acid found in collagen, but it is not the primary contributor to its triple helix structure.

**Clinical Pearl / High-Yield Fact**
Hydroxyproline is an essential amino acid for collagen synthesis, and its deficiency can lead to conditions such as osteogenesis imperfecta, a genetic disorder characterized by fragile bones and poor wound healing.

**Correct Answer:** C. Hydroxyproline