## Core Concept
Bitemporal hemianopia is a type of visual field defect where there is a loss of vision in the outer half of each visual field, i.e., the temporal fields of both eyes. This condition typically results from damage to the **optic chiasm**. The optic chiasm is the part of the visual pathway where the optic nerves from each eye cross over partially.

## Why the Correct Answer is Right
The correct answer, which involves a lesion affecting the optic chiasm, leads to bitemporal hemianopia because the crossing fibers from the nasal hemiretina of each eye are damaged. These fibers carry information from the temporal visual fields. The **optic chiasm** is crucial as it is the site where fibers from the nasal retina of each eye cross over to the opposite side of the brain, while the temporal retinal fibers do not cross. A lesion compressing or damaging the optic chiasm, such as a pituitary tumor, can lead to this specific visual field defect.

## Why Each Wrong Option is Incorrect
- **Option A:** Lesions before the optic chiasm (like those affecting one optic nerve) would cause visual loss in one eye, not bitemporal hemianopia.
- **Option B:** A lesion after the optic chiasm, such as one affecting the optic tract or further downstream, would cause a different type of visual field defect, typically a homonymous hemianopia.
- **Option D:** A lesion affecting the occipital lobe or the entire visual pathway posterior to the optic chiasm would not cause bitemporal hemianopia but could cause cortical blindness or other types of visual field defects.

## Clinical Pearl / High-Yield Fact
A classic cause of bitemporal hemianopia is a **pituitary adenoma**. The pituitary gland is located close to the optic chiasm, and an enlarging pituitary tumor can compress the optic chiasm, leading to this visual field defect. This is a high-yield fact for exams as it connects a specific clinical presentation with a common pathology.

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