**Core Concept**
Insulin is a hormone composed of multiple polypeptide chains, which play a crucial role in glucose regulation in the body. The structure of insulin is vital for its function, and any alterations can lead to various pathologies.

**Why the Correct Answer is Right**
Insulin is composed of two polypeptide chains: the A chain and the B chain, which are linked by disulfide bridges. The A chain has 21 amino acids, and the B chain has 30 amino acids. The combination of these two chains forms the insulin molecule, which is essential for glucose uptake in cells. The A chain is involved in the binding of insulin to its receptor, while the B chain is crucial for the stabilization of the insulin molecule.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because insulin is not composed of a single polypeptide chain. The A and B chains are distinct and play different roles in insulin function.
* **Option B:** This option is incorrect because insulin does not consist of three polypeptide chains. The A and B chains are the primary components of insulin, with no additional chains.
* **Option C:** This option is incorrect because insulin does not consist of five polypeptide chains. The A and B chains are the fundamental components of insulin, and there is no evidence to suggest additional chains.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that insulin's structure is critical for its function. Alterations in the A or B chain can lead to various pathologies, including diabetes mellitus. Understanding the structure of insulin is vital for the development of insulin analogs and the treatment of diabetes.

**Correct Answer: C. Two polypeptide chains.**