**Core Concept**
Bitemporal hemianopia is a type of visual field defect characterized by the loss of vision in the outer half of each visual field. This condition is often associated with lesions or damage to specific areas of the brain, particularly those involved in the visual pathway. The underlying anatomical structure responsible for this defect is the optic chiasm, where the optic nerves from each eye partially cross over and continue as the optic tracts.

**Why the Correct Answer is Right**
Bitemporal hemianopia typically occurs due to lesions or damage affecting the optic chiasm, which can be caused by pituitary tumors, craniopharyngiomas, or other space-occupying lesions. As the optic nerves from each eye partially cross over at the optic chiasm, damage to this area can result in the loss of vision in the outer half of each visual field. The optic chiasm is a critical structure in the visual pathway, and lesions here can lead to bitemporal hemianopia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the location or type of lesion responsible for bitemporal hemianopia. While lesions can occur in various parts of the brain, the optic chiasm is the specific location associated with this type of visual field defect.
**Option B:** This option is not a correct answer because it does not specify the type of lesion or the location in the brain responsible for bitemporal hemianopia. Lesions can occur in various parts of the brain, but the optic chiasm is the specific location associated with this type of visual field defect.
**Option D:** This option is incorrect because it does not specify the type of lesion or the location in the brain responsible for bitemporal hemianopia. Lesions can occur in various parts of the brain, but the optic chiasm is the specific location associated with this type of visual field defect.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that bitemporal hemianopia is often associated with lesions or damage to the optic chiasm, which can be caused by pituitary tumors or other space-occupying lesions. This condition can be a clinical clue to the presence of a pituitary tumor or other space-occupying lesion in the sellar region.

**Correct Answer:** C. Optic chiasm lesions.