**Core Concept**
Pheochromocytoma is a rare neuroendocrine tumor of the adrenal gland that secretes excessive amounts of catecholamines, such as epinephrine and norepinephrine. These hormones can cause a range of symptoms, including paroxysmal (sudden) episodes of hypertension, tachycardia, and other cardiovascular effects.

**Why the Correct Answer is Right**
The most common paroxysmal symptom of pheochromocytoma is indeed headache, which occurs due to the sudden onset of hypertension caused by the excessive release of catecholamines. This leads to a rapid increase in blood pressure, which can cause a severe headache. The mechanism involves the activation of alpha-adrenergic receptors in the blood vessels, leading to vasoconstriction and increased blood pressure. The sudden increase in blood pressure can also cause other symptoms such as palpitations, sweating, and tremors.

**Why Each Wrong Option is Incorrect**
**Option B:** While palpitations are a common symptom of pheochromocytoma, they are not the most common paroxysmal symptom. Palpitations occur due to the stimulation of beta-adrenergic receptors in the heart, leading to an increase in heart rate.
**Option C:** Abdominal pain is not a typical paroxysmal symptom of pheochromocytoma. While abdominal pain can occur in some cases, it is not as common as headache.
**Option D:** Hypotension is actually the opposite of what occurs in pheochromocytoma. The excessive release of catecholamines typically leads to hypertension, not hypotension.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that pheochromocytoma can present with a wide range of symptoms, and the diagnosis should be considered in patients with unexplained episodes of hypertension, tachycardia, or other cardiovascular symptoms.

**✓ Correct Answer: A. Headache**