What is the SINGLE most likely visual abnormality associated with a pituitary tumour?

Bitemporal hemianopia is the most likely visual abnormality associated with a pituitary tumor due to the anatomical position of the pituitary gland and the nearby optic chiasm. The pituitary gland is located at the base of the brain, and when a tumor develops, it can grow upward and compress the optic chiasm. This compression affects the crossing fibers from the nasal (medial) half of each retina, leading to a loss of peripheral vision in both eyes, which creates a characteristic "tunnel vision" effect.

This specific pattern of vision loss is what is recognized as bitemporal hemianopia. The other visual abnormalities have different underlying mechanisms or associations. Homonymous hemianopia usually occurs due to lesions in the optic tract or the occipital lobe, affecting the same field of vision in both eyes. Homonymous upper quadrantanopia is also related to lesions in specific areas of the brain and does not typically arise from pituitary abnormalities. Cortical blindness results from damage to the visual cortex rather than directly from a pituitary tumor.

Understanding the relationship between the site of a pituitary tumor and the optic chiasm is crucial in recognizing why bitemporal hemianopia is the visual abnormality