**Core Concept**
Pheochromocytoma is a rare, catecholamine-secreting tumor of the adrenal gland, leading to episodic hypertension and related symptoms. The symptoms are caused by the excessive release of catecholamines, such as adrenaline and noradrenaline.

**Why the Correct Answer is Right**
The correct answer is **Headache**. Headache is the most common symptom of pheochromocytoma, occurring in approximately 70-80% of patients. This is due to the sudden increase in catecholamines, which cause vasodilation and increased blood flow to the head, leading to headache. The headache is often described as severe, throbbing, and episodic, and may be accompanied by other symptoms such as sweating, palpitations, and nausea.

**Why Each Wrong Option is Incorrect**
* **Option A:** Palpitations are a common symptom of pheochromocytoma, but they are not the most common.
* **Option B:** Hypertension is a hallmark of pheochromocytoma, but it is not a symptom in the classical sense.
* **Option D:** Nausea and vomiting may occur in pheochromocytoma, but they are not as common as headache.

**Clinical Pearl / High-Yield Fact**
The "rule of tens" for pheochromocytoma: 10% are bilateral, 10% are malignant, 10% occur in children, and 10% are associated with other endocrine tumors.

**Correct Answer: C. Headache**