## **Core Concept**
The question tests the understanding of physiological changes in blood supply during exercise, specifically focusing on the redistribution of blood flow to meet increased oxygen and nutrient demands of muscles. This involves knowledge of vascular physiology and the autonomic nervous system's role in regulating blood vessel diameter.

## **Why the Correct Answer is Right**
During exercise, there is a significant increase in the demand for oxygen and nutrients by the muscles. This demand is met by increasing blood flow to the muscles. The correct answer, skeletal muscle, experiences increased blood supply due to several mechanisms:
- **Vasodilation**: Exercise leads to the release of various metabolites (e.g., lactic acid, adenosine, and potassium ions) that cause vasodilation of the arterioles supplying skeletal muscle.
- **Autonomic Nervous System**: The sympathetic nervous system, which typically causes vasoconstriction, has a modified effect during exercise. While it does cause vasoconstriction in some areas (like the gastrointestinal tract), it also allows for vasodilation in skeletal muscle through the release of epinephrine (which has a vasodilatory effect on skeletal muscle arterioles) and by reducing the release of norepinephrine in skeletal muscle vasculature.

## **Why Each Wrong Option is Incorrect**
- **Option A (Brain):** While the brain's blood supply does increase during exercise, this increase is not as pronounced or physiologically significant in terms of total blood flow redistribution as it is in skeletal muscle. The brain's blood flow is tightly regulated and does not significantly increase to meet higher oxygen demands during exercise; instead, it maintains a relatively constant flow.
- **Option B (Gastrointestinal Tract):** Blood flow to the gastrointestinal tract actually decreases during exercise. This decrease is part of the blood flow redistribution that prioritizes oxygen and nutrient delivery to the exercising muscles and the heart.
- **Option C (Kidneys):** Similar to the gastrointestinal tract, renal blood flow decreases during exercise. This decrease allows for the redistribution of blood to the muscles and heart.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that during exercise, blood flow is redistributed from visceral organs to skeletal muscles. This redistribution is crucial for meeting the increased metabolic demands of exercising muscles. A classic example of this redistribution is the decrease in splanchnic blood flow during intense exercise, which can lead to symptoms like nausea.

## **Correct Answer:** . Skeletal muscle.