**Core Concept**
Galactorrhea is the spontaneous flow of milk from the breast, not associated with childbirth or nursing. It is often a symptom of hyperprolactinemia, which can cause various endocrine disorders. The combination of galactorrhea and visual defects suggests a pituitary gland anomaly, specifically a prolactinoma, which secretes excess prolactin.

**Why the Correct Answer is Right**
The correct investigation for a suspected prolactinoma would be a **Pituitary MRI**. This imaging modality provides a detailed visualization of the pituitary gland and surrounding structures, allowing for the detection of a prolactinoma. The MRI can also help differentiate between a prolactinoma and other pituitary tumors or lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum prolactin levels would be elevated in cases of hyperprolactinemia, but this test does not provide a definitive diagnosis of a prolactinoma.
**Option B:** A CT scan of the pituitary gland may be useful in emergency situations, but it is not the preferred imaging modality for visualizing the pituitary gland due to its lower resolution compared to MRI.
**Option C:** A thyroid function test may be relevant in cases of hyperprolactinemia, but it is not directly related to the diagnosis of a prolactinoma.
**Option D:** An ultrasound of the breast may be useful in evaluating breast abnormalities, but it is not suitable for imaging the pituitary gland.

**Clinical Pearl / High-Yield Fact**
In cases of suspected prolactinoma, a high-dose dopamine agonist such as cabergoline can be used as a diagnostic tool to suppress prolactin secretion and potentially shrink the tumor. This can also be a therapeutic option for managing prolactinoma.

**Correct Answer:** C.