**Core Concept**
GLUT4 (Glucose Transporter Type 4) is a transmembrane protein responsible for facilitating glucose uptake in insulin-responsive cells, such as adipocytes and skeletal muscle cells. It plays a crucial role in glucose homeostasis by increasing glucose uptake in response to insulin signaling.

**Why the Correct Answer is Right**
GLUT4 is primarily localized to the intracellular vesicles and the trans-Golgi network of adipocytes and skeletal muscle cells. Upon insulin binding to its receptor, it triggers a signaling cascade that leads to the translocation of GLUT4-containing vesicles to the plasma membrane, increasing glucose uptake. This process is essential for the regulation of blood glucose levels. The correct answer is associated with the insulin-responsive glucose uptake mechanism.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because GLUT4 is not primarily localized to the liver or brain cells, which have different glucose transport mechanisms.
**Option B:** This option is incorrect because GLUT1 is a constitutively active glucose transporter present in red blood cells, neurons, and the placenta, and is not insulin-responsive like GLUT4.
**Option C:** This option is incorrect because GLUT2 is a high-capacity, low-affinity glucose transporter primarily found in the liver and pancreatic beta cells, and is not associated with insulin-stimulated glucose uptake.

**Clinical Pearl / High-Yield Fact**
GLUT4 is a key player in the pathophysiology of type 2 diabetes, where insulin resistance and impaired GLUT4 translocation contribute to hyperglycemia.

**Correct Answer: B. Adipocytes and skeletal muscle cells.**