**Core Concept**
Hair on end appearance is a radiological term used to describe the appearance of the cerebral cortex on a computed tomography (CT) scan or magnetic resonance imaging (MRI) of the brain. This appearance is characterized by a pattern of low-density areas in the white matter, which gives the cortex a "hair on end" appearance due to the contrast between the affected and unaffected areas.

**Why the Correct Answer is Right**
The hair on end appearance is typically seen in cases of cerebral edema, particularly in the context of malignant middle cerebral artery infarction. The edema causes a shift of the cerebral cortex, resulting in a characteristic pattern of low-density areas on CT scans or MRI images. This shift is often accompanied by a mass effect, which can lead to increased intracranial pressure and potentially life-threatening complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydrocephalus is characterized by an accumulation of cerebrospinal fluid in the brain, which can cause ventricular enlargement and increased intracranial pressure. However, it does not typically result in a hair on end appearance on CT scans or MRI images.

**Option B:** Cerebral atrophy is a condition characterized by a loss of brain tissue, which can cause a decrease in brain volume and potentially lead to a "shrunken" appearance on imaging studies. However, it is not typically associated with a hair on end appearance.

**Option C:** Cerebral contusion is a type of traumatic brain injury that can cause bleeding in the brain. While contusions can result in a range of imaging abnormalities, they are not typically associated with a hair on end appearance.

**Clinical Pearl / High-Yield Fact**
The hair on end appearance is a classic sign of malignant cerebral edema, which requires prompt recognition and treatment to prevent further brain damage.

**Correct Answer: D. Malignant middle cerebral artery infarction.**