**Core Concept**
Prolactinoma is a type of pituitary adenoma that secretes excess prolactin, leading to various symptoms including visual defects, breast engorgement, and amenorrhea. The underlying pathophysiology involves the overproduction of prolactin, which can be caused by a tumor or hyperplasia of the lactotroph cells in the pituitary gland.

**Why the Correct Answer is Right**
Serum prolactin is the investigation of choice because it is a direct indicator of prolactin levels in the blood. Elevated prolactin levels can confirm the diagnosis of prolactinoma, which is a common cause of the presenting symptoms. The normal range for serum prolactin is typically less than 25 μg/L in non-pregnant women, and levels above this threshold can suggest prolactinoma. The measurement of serum prolactin is a sensitive and specific test for diagnosing prolactinoma, and it is usually the first step in evaluating patients with suspected hyperprolactinemia.

**Why Each Wrong Option is Incorrect**
**Option A:** CT brain is not the investigation of choice for suspected prolactinoma because it may not detect small tumors or hyperplasia of the lactotroph cells, and it can also expose the patient to radiation.

**Option C:** Biopsy is not typically required for diagnosing prolactinoma because serum prolactin levels can confirm the diagnosis, and biopsy carries a risk of complications such as bleeding or infection.

**Option D:** Mr angiography is not the investigation of choice for suspected prolactinoma because it is more expensive and may not provide additional information compared to serum prolactin measurement.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that hyperprolactinemia can be caused by various factors, including pregnancy, stress, and certain medications. Therefore, it is crucial to consider the patient's medical history and other factors when interpreting serum prolactin levels.

**✓ Correct Answer: B. Serum prolactin. Serum measurement to confirm hyperprolactinemia**