Bitemporal hemianopia is caused by lesions at
**Core Concept**
Bitemporal hemianopia is a type of visual field defect characterized by blindness in the outer half of each visual field. This condition is related to the **anatomy of the visual pathway**, specifically the **optic chiasm**. The optic chiasm is the point where the optic nerves partially cross, allowing for the combination of visual information from both eyes.
**Why the Correct Answer is Right**
The correct answer is related to the location where the optic nerves cross, which is the **optic chiasm**. A lesion at this site, such as a **pituitary tumor**, can compress the crossing optic nerves, leading to bitemporal hemianopia. This is because the fibers from the nasal hemiretina of each eye cross over at the optic chiasm to join the uncrossed fibers from the temporal hemiretina of the opposite eye.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because unilateral lesions of the optic nerve result in unilateral blindness, not bitemporal hemianopia.
**Option B:** Incorrect as lesions affecting the optic tract, which is post-chiasmal, would result in homonymous hemianopia, not bitemporal.
**Option D:** Incorrect because occipital lobe lesions affect the visual cortex and can lead to homonymous hemianopia with macular sparing, not bitemporal hemianopia.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that bitemporal hemianopia is often associated with **pituitary adenomas** due to their location near the optic chiasm. Early detection of this visual field defect can prompt further investigation into possible pituitary lesions.
**Correct Answer:** C. optic chiasm.