## Why Pituitary adenoma is right The structure marked **B** is the optic chiasm. A lesion at the chiasm causes bitemporal hemianopia because the crossing nasal retinal fibers (which correspond to the temporal visual fields of both eyes) are damaged. When a pituitary adenoma grows upward from the sella turcica, it compresses the chiasm from below, affecting the inferior crossing fibers first (which represent the superior temporal quadrants). This produces the classic pattern of superior temporal quadrantanopia progressing to complete bitemporal hemianopia. The patient's presentation of tunnel vision, driving difficulties, and superior temporal field loss with a mass visible on pituitary MRI is pathognomonic for pituitary adenoma. (Guyton & Hall 14e Ch 51; Harrison 21e Ch 32) ## Why each distractor is wrong - **Optic nerve glioma**: Causes monocular vision loss (structure A), not bitemporal hemianopia. The optic nerve is anterior to the chiasm and does not contain crossing fibers. - **Retrobulbar optic neuritis**: Presents with monocular vision loss, pain on eye movement, and relative afferent pupillary defect. Does not produce bitemporal hemianopia and is typically inflammatory/demyelinating, not compressive. - **Posterior cerebral artery occlusion**: Causes homonymous hemianopia (structure C — optic tract/occipital lobe lesion), not bitemporal. The visual field loss is on the same side in both eyes, not temporal in both. **High-Yield:** Bitemporal hemianopia = chiasm lesion until proven otherwise; pituitary adenoma is the classic cause; superior temporal quadrants affected first because the tumor grows upward from below. [cite: Guyton & Hall 14e Ch 51; Harrison 21e Ch 32]
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