**Core Concept**
Pheochromocytoma is a rare, usually benign, tumor of the adrenal gland that produces excessive amounts of catecholamines, such as adrenaline and noradrenaline. The diagnosis of pheochromocytoma requires a combination of clinical suspicion, biochemical testing, and imaging studies.

**Why the Correct Answer is Right**
The correct answer is a 24-hour urine collection for metanephrines. Metanephrines are the major metabolites of catecholamines, and elevated levels are highly specific for pheochromocytoma. This test is considered the gold standard for diagnosis because it provides a comprehensive assessment of catecholamine production over a prolonged period. The test is typically performed in conjunction with plasma free metanephrines, which provides a more rapid and sensitive assessment of catecholamine production.

**Why Each Wrong Option is Incorrect**
**Option A:** 24-hour urine collection for catecholamines is less sensitive than metanephrines and may produce false-negative results due to the short half-life of catecholamines.
**Option B:** Plasma free metanephrines is a useful screening test but may produce false-positive results in patients with certain conditions, such as eating disorders or certain medications.
**Option C:** CT or MRI scans are useful for localizing pheochromocytoma but are not sufficient for diagnosis, as many other conditions can produce similar imaging findings.

**Clinical Pearl / High-Yield Fact**
Remember that pheochromocytoma is a "rule-out" diagnosis, meaning that it requires a high index of suspicion and a thorough evaluation to confirm. Clinical presentation may include hypertension, tachycardia, sweating, and headaches, but these symptoms are non-specific and can be seen in many other conditions.

**Correct Answer:** C.