**Core Concept**
Insulin is a hormone produced by the beta cells of the pancreatic islets that plays a crucial role in glucose metabolism. It regulates blood glucose levels by facilitating glucose uptake in cells and inhibiting glucose production in the liver.

**Why the Correct Answer is Right**
Insulin exerts its effects by binding to the insulin receptor on the surface of cells, particularly in the liver, muscles, and adipose tissue. This binding activates the insulin receptor tyrosine kinase, which triggers a cascade of signaling pathways that ultimately lead to the translocation of glucose transporter type 4 (GLUT4) to the cell surface. GLUT4 is responsible for facilitating the uptake of glucose from the bloodstream into the cells, thereby lowering blood glucose levels.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a known action of insulin. Insulin does not have a direct effect on the kidneys to increase sodium reabsorption.
**Option B:** While insulin does have an anabolic effect, it is not primarily responsible for promoting protein synthesis. Other hormones like growth hormone and testosterone have a more significant role in protein synthesis.
**Option C:** Insulin does not have a direct effect on the autonomic nervous system to increase heart rate.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that insulin resistance, a condition where cells become less responsive to insulin, is a hallmark of type 2 diabetes mellitus. This can lead to hyperglycemia and other metabolic complications if left untreated.

**Correct Answer: A. Facilitates glucose uptake in cells and inhibits glucose production in the liver.**