**Core Concept**
The patient's symptoms of increasing visual loss, breast enlargement, and irregular menses suggest a hormonal imbalance, specifically an excess of prolactin. This condition is known as hyperprolactinemia, which can be caused by a pituitary adenoma secreting prolactin.

**Why the Correct Answer is Right**
The investigation of choice is a **Pituitary MRI**, which can help identify the underlying cause of hyperprolactinemia. A pituitary adenoma, also known as a prolactinoma, can secrete excess prolactin, leading to the patient's symptoms. The MRI can also help determine the size and location of the adenoma, which is crucial for planning treatment. The pituitary gland is located at the base of the brain, and a pituitary MRI is the best imaging modality to visualize this region.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum prolactin level is not the investigation of choice because while it can confirm hyperprolactinemia, it does not identify the underlying cause. A high prolactin level can be due to various factors, including pregnancy, certain medications, or a pituitary adenoma.

**Option B:** CT scan of the pituitary is not the best choice because it may not provide sufficient detail of the pituitary gland and surrounding structures. MRI is superior for visualizing soft tissues and is the preferred imaging modality for pituitary disorders.

**Option C:** Serum estrogen level is not relevant to the patient's symptoms, which are primarily related to hyperprolactinemia and its effects on the pituitary gland.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **hyperprolactinemia can be caused by a variety of factors, including pregnancy, certain medications, and pituitary adenomas**. In the context of a middle-aged female with increasing visual loss, breast enlargement, and irregular menses, a pituitary MRI is the most appropriate investigation to identify the underlying cause.

**Correct Answer:** C.