**Core Concept**
The investigation of choice in parathyroid pathology involves assessing parathyroid hormone (PTH) levels, as well as imaging studies to localize the parathyroid gland(s). **Parathyroid hormone** (PTH) plays a crucial role in maintaining calcium homeostasis through its effects on bone resorption, renal calcium reabsorption, and intestinal calcium absorption.

**Why the Correct Answer is Right**
The investigation of choice in parathyroid pathology is **Selective Parathyroid Venous Sampling (PVS)**. This procedure involves cannulating the parathyroid veins and measuring PTH levels to determine which gland is hyperfunctioning. This is particularly useful in cases of **multiple gland disease** or when **localizing** the abnormal gland is challenging. PVS is considered the gold standard for diagnosing parathyroid pathology, especially in patients with **primary hyperparathyroidism**.

**Why Each Wrong Option is Incorrect**
**Option A:** **Sestamibi Scan** is a useful imaging study for localizing parathyroid adenomas, but it is not the investigation of choice for parathyroid pathology. While it can help identify the location of the abnormal gland, it may not always distinguish between adenoma and hyperplasia.

**Option B:** **Ultrasound** is a non-invasive imaging modality that can be used to evaluate the parathyroid glands, but it is not as sensitive as PVS for detecting parathyroid pathology. Ultrasound may not always accurately identify the location or extent of parathyroid disease.

**Option C:** **CT Scan** is not typically used as a first-line imaging study for parathyroid pathology. While it can provide detailed images of the parathyroid glands, it is not as sensitive as PVS for detecting parathyroid disease.

**Clinical Pearl / High-Yield Fact**
Remember that **Selective Parathyroid Venous Sampling (PVS)** is the investigation of choice for parathyroid pathology, especially in cases of primary hyperparathyroidism or multiple gland disease. This procedure allows for precise localization of the abnormal gland, which is essential for guiding surgical intervention.

**Correct Answer: C. Sestamibi Scan is a useful imaging study for localizing parathyroid adenomas, but it is not the investigation of choice for parathyroid pathology.**