**Core Concept**
Homonymous hemianopia is a type of visual field defect where a patient has partial or complete loss of vision in one half of their visual field on the same side in both eyes. This condition is typically caused by lesions in the optic tracts, optic radiations, or visual cortices of the brain.

**Why the Correct Answer is Right**
The correct answer involves damage to the optic chiasm or the optic tracts. When a lesion affects the optic chiasm, it can cause homonymous hemianopia by interrupting the crossing fibers from the nasal halves of the retina. This disrupts the visual information from the contralateral visual field in both eyes. The optic tracts are responsible for transmitting visual information from the optic chiasm to the lateral geniculate nucleus in the thalamus. Damage to these tracts can also result in homonymous hemianopia.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because a lesion in the optic nerve would cause unilateral visual loss in one eye, not homonymous hemianopia.
**Option B:** This is incorrect because a lesion in the visual cortex would cause homonymous hemianopia, but it is not the most common location for this type of defect.
**Option C:** This is incorrect because a lesion in the oculomotor nerve would cause pupillary dysfunction, ptosis, or extraocular muscle weakness, but not homonymous hemianopia.

**Clinical Pearl / High-Yield Fact**
Lesions in the optic chiasm or optic tracts can cause homonymous hemianopia, which is often associated with other neurological symptoms such as contralateral hemiparesis or speech disturbances.

**Correct Answer:** D.