**Core Concept**
An acute subdural hematoma is a type of traumatic brain injury characterized by the accumulation of blood between the dura mater and the arachnoid membrane, which surrounds the brain. This condition is often caused by severe head trauma, and its diagnosis relies on imaging modalities such as computed tomography (CT) scans.

**Why the Correct Answer is Right**
The classic CT appearance of an acute subdural hematoma is a crescent-shaped hyperdensity that follows the contours of the brain's surface. This is due to the accumulation of fresh blood, which appears bright on CT scans. The hematoma often spreads along the brain's surface, causing it to bulge outward, and may extend over the cerebral convexities. The dura mater's attachment to the falx cerebri and tentorium cerebelli can also help to define the hematoma's location.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it describes the appearance of an epidural hematoma, which is characterized by a biconvex or lens-shaped hyperdensity.

**Option B:** This option is incorrect as it describes the appearance of a subarachnoid hemorrhage, which is characterized by diffuse or patchy hyperdensity in the subarachnoid space.

**Option C:** This option is incorrect as it describes the appearance of a subdural hematoma that is not acute, which may appear as a lower-density area on CT scans.

**Clinical Pearl / High-Yield Fact**
A key feature of acute subdural hematomas is their rapid progression, and they can cause significant mass effect and brain herniation if not promptly treated. Therefore, it is essential to recognize this classic CT appearance to make an accurate diagnosis and initiate timely management.

**Correct Answer:** C