**Core Concept**
Pheochromocytoma is a rare tumor of the adrenal medulla that secretes catecholamines, leading to excessive sympathetic activity. This results in various systemic effects, including hypertension, tachycardia, and sweating.

**Why the Correct Answer is Right**
Pheochromocytomas are known to predominantly secrete norepinephrine (also known as noradrenaline), which is a potent vasoconstrictor. The tumor cells of pheochromocytoma have a higher expression of tyrosine hydroxylase and dopamine beta-hydroxylase enzymes, which are involved in the conversion of dopamine to norepinephrine. As a result, pheochromocytomas typically secrete more norepinephrine than epinephrine, which is primarily secreted by the normal adrenal medulla.

**Why Each Wrong Option is Incorrect**
**Option B:** Epinephrine is the primary catecholamine secreted by the normal adrenal medulla, not pheochromocytomas.
**Option C:** Dopamine is a precursor to norepinephrine and epinephrine, but it is not the primary catecholamine secreted by pheochromocytomas.
**Option D:** VMA (vanillylmandelic acid) is a metabolite of catecholamines, but it is not a catecholamine itself and is not secreted in higher concentrations by pheochromocytomas.

**Clinical Pearl / High-Yield Fact**
Pheochromocytomas can secrete catecholamines intermittently, leading to episodic hypertension and tachycardia. Patients with pheochromocytoma may have a characteristic "hazard sign" on their blood pressure tracing, which shows a sudden increase in blood pressure with each heartbeat.

✓ Correct Answer: A. Norepinephrine