An unconscious 54-year-old female is admitted to the hospital. A CT scan reveals a tumor in her brain, producing a tentorial herniation. When she regains consciousness, her right eye is directed laterally and downward, with complete ptosis of her upper eyelid and pupillary dilation. Which of the following lobes of the brain is affected by the tumor?

## Core Concept
The clinical presentation described indicates a third cranial nerve (oculomotor nerve) palsy, which is often associated with transtentorial herniation due to increased intracranial pressure. The third cranial nerve originates from the midbrain and is responsible for controlling several eye muscles, including those involved in eyelid opening and pupil constriction.

## Why the Correct Answer is Right
The symptoms described—lateral and downward deviation of the eye, complete ptosis (drooping) of the upper eyelid, and pupillary dilation—are characteristic of a third cranial nerve palsy. This nerve palsy occurs because the tumor causes transtentorial herniation, where the uncus of the temporal lobe herniates through the tentorial notch, compressing the third cranial nerve against the tentorium cerebelli. The **temporal lobe** is the structure most commonly associated with this type of herniation and subsequent third nerve palsy.

## Why Each Wrong Option is Incorrect
- **Option A:** The **frontal lobe** is not directly associated with the third cranial nerve palsy symptoms described. While frontal lobe lesions can cause eye movement abnormalities, the specific constellation of symptoms here points more directly to a problem affecting the third cranial nerve.
- **Option B:** The **parietal lobe** is involved in sensory processing and is not typically associated with cranial nerve III palsy or the specific symptoms described.
- **Option D:** The **occipital lobe** is primarily involved in visual processing. Lesions here would more likely cause visual field defects rather than the motor symptoms described.

## Clinical Pearl / High-Yield Fact
A key clinical point to remember is that a **fixed, dilated pupil** in the setting of increased intracranial pressure and impending or actual herniation is an **emergency** that requires immediate intervention. The presence of a third cranial nerve palsy, as suggested by the symptoms in this case, is a critical sign that requires prompt neurosurgical evaluation.

## Correct Answer Line
**Correct Answer: C. Temporal lobe**

✓ Correct Answer: B. Temporal