**Core Concept**
The "hair-on-end" appearance is a radiological finding associated with a specific type of brain lesion. This phenomenon is characterized by the appearance of radiating lines or striations that resemble a "hair-on-end" pattern. It is typically observed on computed tomography (CT) scans or magnetic resonance imaging (MRI) of the brain.

**Why the Correct Answer is Right**
The "hair-on-end" appearance is classically seen in cases of subarachnoid hemorrhage (SAH), particularly when blood has accumulated in the subarachnoid space and is causing a mass effect. This leads to the displacement of the brain parenchyma and the formation of radiating lines or striations. The blood in the subarachnoid space also causes the pia mater to become stretched and irregular, resulting in the characteristic "hair-on-end" appearance. The presence of blood in the subarachnoid space can be due to an aneurysm rupture or other vascular malformations.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the "hair-on-end" appearance is not typically associated with cerebral edema, which would more likely result in a "swiss cheese" or "bubbly" appearance on imaging.
**Option B:** This option is incorrect because demyelinating diseases such as multiple sclerosis do not typically present with a "hair-on-end" appearance on imaging.
**Option C:** This option is incorrect because cerebral vasculitis may present with a variety of imaging findings, including infarcts and hemorrhages, but is not typically associated with a "hair-on-end" appearance.

**Clinical Pearl / High-Yield Fact**
The "hair-on-end" appearance is a classic radiological finding in subarachnoid hemorrhage and should be suspected in any patient presenting with sudden headache, nausea, and vomiting, particularly if they have a history of hypertension or vascular risk factors.

**Correct Answer:** D. Subarachnoid hemorrhage.