**Core Concept**
The insulin receptor is a transmembrane receptor tyrosine kinase that plays a crucial role in glucose metabolism and insulin signaling. It is composed of four subunits: alpha (α), beta (β), and two identical beta-subunits (β) that form the active site.

**Why the Correct Answer is Right**
The insulin receptor consists of a single polypeptide chain that spans the cell membrane, with the extracellular domain containing the ligand-binding site and the intracellular domain containing the tyrosine kinase activity. Upon insulin binding, the receptor undergoes a conformational change, leading to the activation of the tyrosine kinase domain. This activation triggers a cascade of downstream signaling events that regulate glucose uptake, glycogen synthesis, and other metabolic processes.

**Why Each Wrong Option is Incorrect**
* **Option A:** The insulin receptor is not composed of multiple polypeptide chains or subunits.
* **Option B:** The correct number of subunits in the insulin receptor is not two, but rather four: alpha (α), beta (β), and two identical beta-subunits (β).
* **Option D:** The insulin receptor does not have five parts; it consists of a single polypeptide chain with four distinct subunits.

**Clinical Pearl / High-Yield Fact**
The insulin receptor has a high affinity for insulin, with a dissociation constant (Kd) of approximately 10-8 M. This high affinity allows insulin to effectively regulate glucose metabolism in the body.

**Correct Answer: C. 4**