**Core Concept**
Pheochromocytoma is a rare, catecholamine-secreting tumor of the adrenal gland, leading to episodic hypertension, tachycardia, and other systemic effects. The characteristic symptoms are related to the sudden release of catecholamines, such as adrenaline and noradrenaline.

**Why the Correct Answer is Right**
The most characteristic symptom of pheochromocytoma is **hypertensive crisis**, which occurs due to the sudden release of catecholamines. This leads to a massive increase in blood pressure, often accompanied by tachycardia, palpitations, and sweating. The sympathetic nervous system's "fight or flight" response is triggered, resulting in increased heart rate, blood pressure, and cardiac output.

**Why Each Wrong Option is Incorrect**
* **Option A:** Headache is a common symptom, but it's not the most characteristic symptom. Headache can occur due to the increase in blood pressure, but it's not as specific or diagnostic as hypertensive crisis.
* **Option B:** Palpitations are a symptom of pheochromocytoma, but they are not as distinctive as hypertensive crisis. Palpitations can be caused by many conditions, including anxiety, hyperthyroidism, and cardiac arrhythmias.
* **Option C:** Sweating is a symptom of pheochromocytoma, but it's not as specific or diagnostic as hypertensive crisis. Sweating can occur due to many conditions, including anxiety, hyperthyroidism, and autonomic nervous system dysfunction.

**Clinical Pearl / High-Yield Fact**
The "rule of 10s" for pheochromocytoma states that 10% of cases are bilateral, 10% are malignant, and 10% occur in children. Additionally, pheochromocytomas can occur in multiple endocrine neoplasia (MEN) syndromes, such as MEN2A and MEN2B.

**Correct Answer: C. Sweating is a symptom of pheochromocytoma, but it's not as specific or diagnostic as hypertensive crisis. Sweating can occur due to many conditions, including anxiety, hyperthyroidism, and autonomic nervous system dysfunction.**