**Core Concept**
Homonymous hemianopia refers to a type of visual field defect where patients lose half of their visual field on the same side in both eyes. This occurs due to a lesion affecting the optic tract, optic radiation, or the occipital lobe, which are all part of the visual pathway. The optic chiasma, however, is responsible for crossing fibers from each eye, leading to bitemporal hemianopia when affected.

**Why the Correct Answer is Right**
A lesion at the optic chiasma typically affects the crossing nasal fibers from each eye, resulting in bitemporal hemianopia, where patients lose the outer half of their visual field in both eyes. This is because the optic chiasma is where the nasal fibers from each eye cross over to the opposite side, and damage to this area disrupts this crossing. In contrast, homonymous hemianopia involves the same half of the visual field being affected in both eyes, which is characteristic of lesions in the optic tract, optic radiation, or the occipital lobe.

**Why Each Wrong Option is Incorrect**
**Option B:** A lesion in the optic tract would indeed cause homonymous hemianopia, as the uncrossed fibers from each eye are affected, leading to the same half of the visual field being lost in both eyes.
**Option C:** Similarly, a lesion in the optic radiation would cause homonymous hemianopia due to the involvement of the uncrossed fibers from each eye.
**Option D:** A lesion in the occipital lobe would also result in homonymous hemianopia, as the primary visual cortex is affected, leading to a loss of the same half of the visual field in both eyes.

**Clinical Pearl / High-Yield Fact**
When assessing patients with visual field defects, it's essential to determine the type of hemianopia (homonymous or bitemporal) to localize the lesion in the visual pathway. This can help guide further investigation and management.

**✓ Correct Answer: A. Optic chiasma**