Final answer:
A patient with tunnel vision is likely experiencing 'bitemporal hemianopsia', caused by a pituitary adenoma pressing on the optic chiasm, resulting in the loss of peripheral vision. So, the correct option is C.
Step-by-step explanation:
When a patient experiences visual disturbances such as an abnormal peripheral vision, often described as tunnel vision, this condition suggests that there is a problem with the patient's visual field. This tunnel vision effect typically does not affect the superior and inferior visual fields but rather indicates a loss of the peripheral vision mainly to the right and left sides.
In cases where a pituitary adenoma is involved, the tumor may be pressing on the optic chiasm, the point where the optic nerves from both eyes meet and where fibers cross over. This is due to the tumor's location in the sella turcica of the sphenoid bone, positioned directly inferior to the optic chiasm. The pressing of a pituitary adenoma against this area affects the fibers conveying information from the lateral or peripheral visual field (originating from the medial retina of either eye), thus causing a characteristic loss of vision.
Based on the symptoms described, the most likely diagnosis is C. bitemporal hemianopsia due to pressure exerted on the optic chiasm by a pituitary adenoma, causing loss of the outermost areas of visual perception and the inability to see objects on the far right and left, thus leading to tunnel vision.