**Core Concept**
A lesion at the optic chiasm involves the crossing of the nasal fibers of the optic nerves from each eye. This crossing results in bitemporal hemianopia due to loss of nasal retinal input from both eyes, leading to a visual field defect where the temporal visual fields are lost on both sides.

**Why the Correct Answer is Right**
At the optic chiasm, nasal retinal fibers from each eye cross to the opposite side. A lesion at the chiasm disrupts the transmission of nasal retinal fibers, causing loss of the temporal visual fields in both eyes. This results in bitemporal hemianopia — a defect where patients see only the nasal half of the visual field in both eyes, with no peripheral vision in the temporal zones.

**Why Each Wrong Option is Incorrect**
Option A: Homonymous hemianopia occurs due to lesions in the optic tract or lateral geniculate nucleus, not at the chiasm. It affects the same half of the visual field in both eyes, which is not seen in chiasmal lesions.
Option C: Upper quadrantanopia is seen in lesions of the superior optic tract or posterior cerebral artery, not at the chiasm.
Option D: Lower quadrantanopia results from lesions in the inferior optic tract or occipital lobe, unrelated to chiasmal pathology.

**Clinical Pearl / High-Yield Fact**
Remember: **"Nasal fibers cross at the chiasm"** — loss of nasal retinal input leads to **bitemporal hemianopia**. This is a classic exam trap — do not confuse it with homonymous hemianopia or quadrantanopia.

✓ Correct Answer: B. Bitemporal hemianopia