Final answer:
The 45-year-old woman most likely has a Pituitary macroadenoma, indicated by her symptoms and visual field loss. A pituitary tumor pressing against the optic chiasm explains her bilateral hemianopia.
Step-by-step explanation:
The most likely diagnosis for a 45-year-old woman with bipolar disorder, who is presenting with amenorrhea, galactorrhea, decreased libido, anorgasmia, and an elevated serum prolactin level while on risperidone, and decreased vision in both lateral visual fields is C. Pituitary macroadenoma. The symptoms of excessive prolactin production, including amenorrhea and galactorrhea, coupled with the specific pattern of visual field loss, known as bilateral hemianopia, point towards a pituitary tumor. This is due to the pituitary macroadenoma pressing against the optic chiasm, which causes interference with the signal transmission, leading to peripheral vision loss.
A pituitary tumor can cause perceptual losses in the lateral visual field because the pituitary gland is located directly inferior to the hypothalamus, near the optic chiasm. This pressing against the optic chiasm by a growth such as a macroadenoma typically results in the loss of lateral peripheral vision while leaving the superior and inferior fields intact.