Final answer:
The most likely visual field defect from compression of the optic chiasm by a pituitary tumor is the loss of the temporal visual fields in both eyes, known as bitemporal hemianopia.
Step-by-step explanation:
A patient comes in with a tumor of the pituitary gland, which grows upwards into the optic chiasm and causes a visual field defect. The optic nerves from both eyes meet at the optic chiasm where some nerve fibers cross over to the opposite side. This structure is crucial for proper visual processing, with fibers from the nasal retina (which captures the temporal visual field) decussating to the contralateral hemisphere of the brain. When the pituitary gland, located in the sella turcica just below the optic chiasm, develops a tumor, it can compress these crossing fibers. This results in a loss of peripheral vision known as bitemporal hemianopia. Therefore, the most likely visual field defect from compression of the optic chiasm is loss of the temporal visual fields in both eyes (D).