## Core Concept
McEwan's pupil, also known as Hutchinson's pupil or Hutchinson's sign in some contexts, refers to a clinical sign observed in patients with severe brain injury or cerebral herniation. It is characterized by a dilated pupil on the side of a compressive or expansive intracranial lesion.

## Why the Correct Answer is Right
The correct answer, **D. Transtentorial herniation**, is associated with McEwan's pupil because this condition involves the herniation of brain tissue through the tentorial notch. This herniation compresses the oculomotor nerve (cranial nerve III), leading to ipsilateral pupillary dilation due to the interruption of parasympathetic fibers that run along the surface of the third nerve. This compression results in a loss of parasympathetic tone, causing the pupil to dilate.

## Why Each Wrong Option is Incorrect
- **Option A:** Not specified, but generally, other types of herniation or brain injuries do not cause this specific sign.
- **Option B:** Not specified, but conditions not directly affecting the oculomotor nerve or leading to transtentorial herniation would not produce McEwan's pupil.
- **Option C:** Not specified, but similar to options A and B, without direct involvement of the third cranial nerve or transtentorial herniation, McEwan's pupil would not be observed.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the presence of a dilated pupil (McEwan's pupil) in the setting of head trauma or increased intracranial pressure is a critical sign indicating impending or actual transtentorial herniation, necessitating immediate medical intervention. This sign is a classic example of how neurology and neurosurgery intersect with emergency medicine.

## Correct Answer: D. Transtentorial herniation