## **Core Concept**
The question revolves around a patient presenting with episodic hypertension, headache, and a thyroid nodule. These symptoms suggest a possible diagnosis of **pheochromocytoma**, a tumor of the adrenal glands that leads to excessive production of catecholamines (like adrenaline and noradrenaline), causing episodic hypertension and headaches. The presence of a thyroid nodule might hint at **Multiple Endocrine Neoplasia (MEN) syndromes**, specifically MEN2, which associates pheochromocytoma with medullary thyroid carcinoma.

## **Why the Correct Answer is Right**
The investigation of choice for pheochromocytoma, which seems to be the primary concern given the symptoms, is **24-hour urine collection for metanephrines or plasma free metanephrines**. This is because pheochromocytomas produce catecholamines (adrenaline and noradrenaline) and their metabolites (metanephrines). Direct measurement of plasma free metanephrines or urinary fractionated metanephrines is highly sensitive for diagnosing pheochromocytoma. Imaging like CT or MRI of the abdomen may be used subsequently to localize the tumor but is not the initial diagnostic step.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a **CT scan of the abdomen** can help localize an adrenal mass suggestive of pheochromocytoma, it is not the initial diagnostic test for confirming the biochemical diagnosis of pheochromocytoma.
- **Option B:** **Thyroid function tests (TFTs)** might be useful given the thyroid nodule but do not directly address the cause of episodic hypertension and headaches.
- **Option C:** Although **Ultrasound of the thyroid** can evaluate the thyroid nodule, it does not help in diagnosing the cause of episodic hypertension and headache.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is to always consider pheochromocytoma in cases of episodic hypertension, especially when accompanied by symptoms like headache, palpitations, and sweating. Also, remember that **pheochromocytomas are associated with MEN2 syndrome**, which also includes medullary thyroid carcinoma and primary hyperparathyroidism. Therefore, screening for other components of MEN syndromes may be warranted if pheochromocytoma is diagnosed.

## **Correct Answer:** . Plasma free metanephrines.