## Core Concept
GLUT 4, or Glucose Transporter Type 4, is a protein that plays a crucial role in glucose metabolism. It is a facilitated glucose transporter that is primarily responsible for insulin-regulated glucose uptake in cells. GLUT 4 is known for its presence in tissues that are sensitive to insulin.

## Why the Correct Answer is Right
The correct answer, , indicates that GLUT 4 is present in adipose tissue and skeletal muscle. This is accurate because GLUT 4 is predominantly found in adipocytes (fat cells) and skeletal muscle cells. In these tissues, GLUT 4 translocates to the cell surface in response to insulin signaling, facilitating the uptake of glucose from the bloodstream into these cells. This process is vital for regulating blood glucose levels, particularly after a meal.

## Why Each Wrong Option is Incorrect
- **Option A:** - This option is incorrect because, although the endothelium does have glucose transporters, GLUT 4 is specifically noted for its role in adipose tissue and skeletal muscle, not primarily in endothelial cells.
- **Option B:** - This option is incorrect because GLUT 1 is the primary glucose transporter in red blood cells, not GLUT 4.
- **Option C:** - This option might seem plausible since the brain does have glucose transporters, but GLUT 4 is not the primary type found in brain tissue; GLUT 1 is more prevalent in the blood-brain barrier.

## Clinical Pearl / High-Yield Fact
A key point to remember is that GLUT 4 is crucial for glucose homeostasis and is a major target for insulin action. Impaired GLUT 4 function or translocation is associated with insulin resistance, a hallmark of type 2 diabetes. Understanding the role of GLUT 4 in glucose metabolism is essential for managing and understanding the pathophysiology of diabetes.

## Correct Answer: . Adipose tissue and skeletal muscle.