**Core Concept**
Pheochromocytomas are rare, catecholamine-secreting tumors arising from the adrenal medulla, and they predominantly secrete catecholamines such as epinephrine (adrenaline) and norepinephrine. This excessive hormone secretion leads to various systemic symptoms and complications.

**Why the Correct Answer is Right**
Pheochromocytomas originate from chromaffin cells, which are neuroendocrine cells responsible for producing catecholamines. The excessive secretion of epinephrine and norepinephrine results from the abnormal activation of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine synthesis. This leads to increased levels of these hormones in the bloodstream, causing symptoms like hypertension, tachycardia, and palpitations.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because pheochromocytomas do not predominantly secrete vasopressin, a hormone involved in water reabsorption in the kidneys.

**Option B:** This option is incorrect because calcitonin, a hormone secreted by the thyroid gland, is not a primary product of pheochromocytomas.

**Option C:** This option is incorrect because gastrin, a hormone involved in gastric acid secretion, is not a hormone secreted by pheochromocytomas.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that pheochromocytomas are often associated with genetic syndromes, such as multiple endocrine neoplasia type 2 (MEN2) and von Hippel-Lindau disease (VHL), and should be suspected in patients with a family history of these conditions.

**Correct Answer: A. Epinephrine (adrenaline).**