**Core Concept:** Insulin is a hormone produced by the pancreas that plays a crucial role in regulating blood glucose levels by facilitating glucose uptake into cells. One of its effects is to stimulate glucose transporters, specifically GLUT4, to increase glucose uptake in target tissues like muscle, fat, and liver.

**Why the Correct Answer is Right:** GLUT4 (Glucose Transporter Type 4) is a glucose transporter protein that plays a vital role in insulin-stimulated glucose uptake. In the presence of insulin, GLUT4 is translocated from intracellular vesicles to the cell membrane, allowing glucose to enter the cell. This process is essential for maintaining euglycemia and preventing hyperglycemia.

**Why Each Wrong Option is Incorrect:**

A. GLUT2 (Glucose Transporter Type 2) is primarily involved in glucose uptake in the liver and renal tubules, not insulin-stimulated glucose uptake.
B. GLUT1 (Glucose Transporter Type 1) is primarily involved in facilitating glucose uptake in the brain, not insulin-stimulated glucose uptake in peripheral tissues.
C. GLUT3 (Glucose Transporter Type 3) is involved in glucose uptake in the brain, not insulin-stimulated glucose uptake in peripheral tissues.
D. GLUT3 (Glucose Transporter Type 3) is involved in glucose uptake in the brain, not insulin-stimulated glucose uptake in peripheral tissues.

**Clinical Pearl:** Understanding the role of GLUT4 in insulin-stimulated glucose uptake is crucial for understanding the pathophysiology of diabetes mellitus, as impaired insulin signaling and GLUT4 translocation can lead to hyperglycemia. This knowledge can guide treatment strategies, such as recommending exercise or dietary changes to improve insulin sensitivity and GLUT4 function in type 2 diabetics.

**Correct Answer:** D. GLUT3 (Glucose Transporter Type 3) is involved in glucose uptake in the brain, not insulin-stimulated glucose uptake in peripheral tissues.