**Core Concept**
The clinical presentation suggests a possible endocrine disorder, particularly one involving the **hypothalamic-pituitary** axis, given the symptoms of visual loss, breast enlargement (**galactorrhea**), and irregular menses (**oligomenorrhea** or **amenorrhea**).

**Why the Correct Answer is Right**
Although the correct answer choice is missing, the investigation of choice for such symptoms would typically involve assessing the function of the pituitary gland, considering its role in regulating various endocrine functions, including menstruation and lactation. A key diagnostic tool would be **imaging of the pituitary gland**, such as an **MRI**, to identify any abnormalities like tumors (**prolactinoma**) that could explain the patient's symptoms by affecting **prolactin** levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific details of each choice, a general approach would be that options not involving direct assessment of pituitary function or its effects (like hormone level tests or imaging) would be less relevant.
**Option B:** Similarly, this would be incorrect if it doesn't directly relate to diagnosing pituitary disorders.
**Option C:** This option would be wrong if it doesn't involve a direct method for diagnosing the suspected underlying condition, such as a pituitary tumor.
**Option D:** Incorrect if it's not focused on the pituitary gland or its hormonal output.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **galactorrhea** and **amenorrhea** can be symptoms of a **prolactinoma**, a type of pituitary tumor that secretes **prolactin**, and **MRI of the pituitary** is a crucial diagnostic step.

**Correct Answer:** D. MRI of the pituitary gland.