**Core Concept**
The patient's presentation suggests a hormonal imbalance affecting the pituitary gland, leading to visual disturbances and endocrine abnormalities. The symptoms of galactorrhea, menstrual irregularity, and bitemporal superior quadrantopia are characteristic of a pituitary tumor affecting the optic chiasm.

**Why the Correct Answer is Right**
The patient's symptoms are likely due to a prolactinoma, a type of pituitary adenoma secreting excessive prolactin. The bitemporal superior quadrantopia is caused by the tumor pressing on the optic chiasm, while the galactorrhea and menstrual irregularity are due to hyperprolactinemia. The primary optic atrophy is a result of chronic compression of the optic nerves.

* The prolactinoma is likely to be a microadenoma, as it is the most common type of pituitary tumor and is often asymptomatic until it grows large enough to compress surrounding structures.
* The tumor's location near the optic chiasm explains the visual field defects, which are characteristic of pituitary tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** Cushing's disease is caused by an ACTH-secreting pituitary tumor, which would present with symptoms such as weight gain, hypertension, and glucose intolerance, but not galactorrhea or bitemporal superior quadrantopia.

**Option B:** Non-functioning pituitary adenoma would present with symptoms such as visual field defects and headaches, but not galactorrhea or menstrual irregularity.

**Option C:** Craniopharyngioma is a tumor that occurs in the suprasellar region and can present with visual disturbances, but it is not typically associated with galactorrhea or menstrual irregularity.

**Option D:** Empty sella syndrome is a condition where the sella turcica is partially or completely filled with cerebrospinal fluid, but it is not typically associated with galactorrhea or menstrual irregularity.

**Clinical Pearl / High-Yield Fact**
Prolactinomas are often associated with galactorrhea and menstrual irregularity, and can cause visual disturbances by compressing the optic chiasm. Remember to consider pituitary tumors in patients with endocrine abnormalities and visual field defects.

**Correct Answer:** C. Prolactinoma. Prolactinoma is the most likely diagnosis in this case, given the patient's symptoms of galactorrhea, menstrual irregularity, and bitemporal superior quadrantopia.