**Core Concept**
Pheochromocytoma is a rare neuroendocrine tumor of the adrenal gland that produces excessive amounts of catecholamines, leading to hypertension, tachycardia, and other systemic symptoms. The primary pathological mechanism involves the overproduction of catecholamines, such as epinephrine and norepinephrine, which are potent vasoconstrictors and stimulants of the sympathetic nervous system.

**Why the Correct Answer is Right**
The correct answer is **D.** Metabolic acidosis. Pheochromocytoma typically presents with metabolic alkalosis due to the excessive production of catecholamines, which leads to increased renal excretion of hydrogen ions and retention of bicarbonate. This results in an alkalotic metabolic state, characterized by elevated serum bicarbonate levels and decreased serum chloride levels.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hypokalemia. Pheochromocytoma can cause hypokalemia due to the excessive production of aldosterone, a mineralocorticoid hormone that promotes potassium excretion.
* **Option B:** Hyperglycemia. Pheochromocytoma can cause hyperglycemia due to the excessive production of catecholamines, which can stimulate hepatic gluconeogenesis and inhibit insulin release.
* **Option C:** Hypocalcemia. Pheochromocytoma can cause hypocalcemia due to the excessive production of catecholamines, which can lead to decreased parathyroid hormone secretion and increased calcium excretion.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to remember that pheochromocytoma typically presents with a triad of hypertension, tachycardia, and sweating, but can also cause metabolic alkalosis, hypokalemia, and hyperglycemia due to the excessive production of catecholamines.

**Correct Answer: D. Metabolic acidosis.**