**Core Concept**
Pheochromocytoma is a rare, catecholamine-secreting tumor that originates from the chromaffin cells of the adrenal medulla. This tumor is characterized by excessive production of epinephrine (adrenaline) and norepinephrine, leading to various systemic symptoms.

**Why the Correct Answer is Right**
Pheochromocytomas are derived from the chromaffin cells of the adrenal medulla, which are part of the sympathetic nervous system. These cells are responsible for producing and storing catecholamines, such as epinephrine and norepinephrine, which play a crucial role in the body's fight-or-flight response. The excessive production of these hormones by the tumor leads to the characteristic symptoms of pheochromocytoma, including hypertension, tachycardia, and sweating.

**Why Each Wrong Option is Incorrect**
**Option A:** Pancreas - Incorrect because pheochromocytomas are not associated with the pancreas, which produces hormones such as insulin and glucagon.

**Option B:** Thyroid gland - Incorrect because pheochromocytomas are not related to the thyroid gland, which produces hormones such as thyroxine (T4) and triiodothyronine (T3).

**Option C:** Pituitary gland - Incorrect because pheochromocytomas are not regulated by the pituitary gland, which produces hormones such as adrenocorticotropic hormone (ACTH) and thyroid-stimulating hormone (TSH).

**Clinical Pearl / High-Yield Fact**
Remember that pheochromocytomas are often described as the "10% tumor" because they occur in 10% of patients with a family history of the disease, and 10% of patients with a pheochromocytoma have a family history of the disease.

**Correct Answer:** C. Adrenal medulla.