**Core Concept**
The investigation of choice in parathyroid pathology involves identifying the underlying cause of parathyroid gland dysfunction, which can lead to hyperparathyroidism. This condition is characterized by an imbalance in calcium and phosphorus metabolism, often due to parathyroid hormone (PTH) overproduction.

**Why the Correct Answer is Right**
The correct investigation of choice is **Sestamibi scan**, a nuclear medicine imaging technique that uses Technetium-99m sestamibi to localize parathyroid adenomas or hyperplastic glands. This scan takes advantage of the increased mitochondrial activity in abnormal parathyroid tissues, allowing for accurate identification and localization of parathyroid pathology. The sestamibi scan is particularly useful in differentiating parathyroid adenomas from thyroid nodules and other thyroid pathology.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is useful in identifying parathyroid glands and guiding fine-needle aspiration biopsy but is not the investigation of choice for parathyroid pathology.
**Option B:** CT scan and MRI are useful in evaluating the extent of parathyroid pathology and its relationship to surrounding structures but are not the primary investigation of choice.
**Option C:** Serum calcium and PTH levels are essential in diagnosing hyperparathyroidism but are not an imaging technique and do not localize parathyroid pathology.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with hyperparathyroidism, it is essential to consider the possibility of familial hyperparathyroidism, particularly in cases with a family history of the condition.

**Correct Answer: C. Sestamibi scan**