**Core Concept**
During aerobic exercise, the body adapts to meet the increased oxygen demand of working muscles while maintaining vital functions. The brain, being a critical organ, requires a constant blood supply to ensure proper function and prevent ischemia.

**Why the Correct Answer is Right**
The brain's blood flow remains relatively constant during aerobic exercise due to the unique autoregulation mechanism. This is mediated by the autonomic nervous system, which adjusts the diameter of cerebral blood vessels to maintain a stable blood pressure and flow. The brain's high energy demand and limited storage capacity for oxygen and glucose necessitate this constant blood supply. The autoregulation mechanism ensures that the brain receives approximately 15% of the cardiac output, even during periods of increased physical activity.

**Why Each Wrong Option is Incorrect**
**Option B:** The heart, being a muscular organ, requires increased blood flow during aerobic exercise to meet the oxygen demand of its own muscle cells.
**Option C:** The kidneys, although vital for maintaining electrolyte balance and blood pressure, exhibit a decrease in blood flow during exercise due to the redistribution of blood to working muscles.
**Option D:** Skeletal muscle, the primary site of aerobic exercise, experiences a significant increase in blood flow to deliver oxygen and nutrients for energy production.

**Clinical Pearl / High-Yield Fact**
The brain's autoregulation mechanism is essential for maintaining cognitive function during exercise. This is particularly important for individuals with pre-existing cardiovascular conditions or those who are physically deconditioned, as the brain's blood supply can be compromised during periods of increased physical activity.

**✓ Correct Answer: A. Brain**