**Core Concept**
Hyperprolactinemia is a condition characterized by elevated levels of prolactin in the blood, which can be caused by various factors including pituitary tumors, thyroid disorders, and certain medications. The investigation of choice for hyperprolactinemia aims to identify the underlying cause and assess the severity of the condition.

**Why the Correct Answer is Right**
The investigation of choice for hyperprolactinemia is a **Pituitary MRI**. This imaging modality is essential in identifying pituitary tumors, such as prolactinomas, which are the most common cause of hyperprolactinemia. A pituitary MRI can also help assess the size and location of the tumor, which is crucial in determining the appropriate treatment plan.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum prolactin level is a useful screening test, but it is not the investigation of choice for hyperprolactinemia. Elevated serum prolactin levels can be seen in various conditions, and a pituitary MRI is needed to identify the underlying cause.
**Option B:** Thyroid function tests (TFTs) may be ordered to evaluate thyroid disorders that can cause hyperprolactinemia, but they are not the primary investigation of choice.
**Option C:** A complete blood count (CBC) is not directly related to the evaluation of hyperprolactinemia and is not the investigation of choice.

**Clinical Pearl / High-Yield Fact**
A pituitary MRI is essential in differentiating between microadenomas (less than 10 mm) and macroadenomas (larger than 10 mm), which have different treatment implications.

**Correct Answer:** C.