**Core Concept**
The anterior cerebral artery (ACA) is a crucial branch of the internal carotid artery that supplies a significant portion of the brain. It plays a vital role in supplying blood to the medial surface of the brain, which includes the medial aspects of the frontal and parietal lobes.

**Why the Correct Answer is Right**
The ACA primarily supplies the medial aspect of the brain, including the anterior and medial walls of the frontal lobe, the medial wall of the parietal lobe, and the corpus callosum. This area is responsible for various higher-order functions, including motor control, sensory processing, and cognitive functions. The ACA's territory also includes the anterior cingulate gyrus, which is involved in emotion regulation, error detection, and motivation.

**Why Each Wrong Option is Incorrect**
* **Option A:** The anterior cerebral artery does not supply the anterior aspect of the brain. While it does supply the anterior wall of the frontal lobe, this is not the primary territory of the ACA.
* **Option B:** The lateral surface of the brain is primarily supplied by the middle cerebral artery, not the anterior cerebral artery.
* **Option D:** The posterior aspect of the brain is supplied by the posterior cerebral artery, not the anterior cerebral artery.

**Clinical Pearl / High-Yield Fact**
The ACA is also known as the "leg artery" because it supplies the motor and sensory areas of the lower limb. Damage to the ACA can result in weakness or paralysis of the lower limb, making it an important area to consider in the diagnosis of stroke or other vascular disorders.

✓ Correct Answer: C. Medial. The anterior cerebral artery supplies the medial aspect of the brain.