**Core Concept**
Pheochromocytoma is a rare, usually benign, tumor of the adrenal medulla that secretes excessive catecholamines (epinephrine and norepinephrine). The symptoms and signs of pheochromocytoma are due to the excessive release of these hormones into the bloodstream.

**Why the Correct Answer is Right**
The commonest symptom of pheochromocytoma is hypertension, which occurs due to the vasoconstrictive effects of norepinephrine. **Norepinephrine** causes blood vessels to constrict, leading to an increase in blood pressure. Additionally, the excessive release of epinephrine can also contribute to hypertension by increasing cardiac output and peripheral resistance. The episodic nature of catecholamine release can lead to paroxysmal hypertension, which is a hallmark of pheochromocytoma.

**Why Each Wrong Option is Incorrect**
**Option A:** Headache - While headache can be a symptom of pheochromocytoma, it is not the most common symptom. Headache is often associated with the episodic hypertension caused by catecholamine release.
**Option B:** Sweating - Sweating is a symptom of pheochromocytoma, but it is not the most common symptom. Sweating is often accompanied by other symptoms such as palpitations and hypertension.
**Option C:** Palpitations - Palpitations are a symptom of pheochromocytoma, but they are not the most common symptom. Palpitations are often associated with the excessive release of epinephrine and norepinephrine.

**Clinical Pearl / High-Yield Fact**
The "rule of tens" is a useful mnemonic for remembering the symptoms of pheochromocytoma: **ten percent** of patients have a family history, **ten percent** have multiple endocrine neoplasia (MEN) syndrome, and **ten percent** have a history of other tumors. Additionally, **ten percent** of patients have paroxysmal hypertension, and **ten percent** have a palpable abdominal mass.

**Correct Answer: C. Palpitations**