Which transporter is NOT primarily found in the small intestine?

GLUT3 is predominantly expressed in neurons and plays a significant role in glucose uptake in the brain. This transporter is highly efficient in facilitating glucose transport even at low glucose concentrations, which is vital for the functioning of nerve cells that rely heavily on glucose as their primary energy source.

In contrast, the other transporters listed have primary roles in the small intestine. GLUT1 is involved in glucose transport across the blood-brain barrier and is also expressed in various tissues including the small intestine. GLUT2, found on the basolateral membrane of enterocytes, facilitates the transport of glucose, galactose, and fructose into the bloodstream after digestion. GLUT4 is primarily found in adipose tissue and muscle, where it is regulated by insulin, but it can also be expressed in small amounts in the small intestine.

Hence, GLUT3's primary location and function in neural tissue rather than in the small intestine establishes it as the correct answer in distinguishing which transporter is not principally associated with that specific area of the gastrointestinal tract.