## Core Concept
The question tests knowledge of imaging modalities used for detecting extra-adrenal pheochromocytomas, also known as paragangliomas. Pheochromocytomas and paragangliomas are neuroendocrine tumors that can produce excessive catecholamines, leading to hypertension and other symptoms. Extra-adrenal pheochromocytomas are rarer and can be found in various locations along the sympathetic nervous system.

## Why the Correct Answer is Right
**Metaiodobenzylguanidine (MIBG) scintigraphy**, labeled as , is particularly sensitive for detecting neuroendocrine tumors like pheochromocytomas and paragangliomas because MIBG is taken up by the norepinephrine transporter, which is present in these cells. This specificity makes MIBG highly sensitive for extra-adrenal pheochromocytomas, which may not be easily detected by other imaging modalities due to their variable locations and sometimes small size.

## Why Each Wrong Option is Incorrect
* **Option A:** CT scans are useful for detecting pheochromocytomas and can provide anatomical details but are not as specific or sensitive as MIBG for neuroendocrine tumors.
* **Option B:** MRI can be used, especially for adrenal pheochromocytomas, but it's not as sensitive as MIBG for extra-adrenal locations.
* **Option D:** Ultrasound is less commonly used for detecting pheochromocytomas, especially for extra-adrenal tumors, due to limited depth penetration and variable tumor location.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **MIBG scintigraphy** is not only diagnostic but can also be used for staging and for assessing the response to treatment in patients with pheochromocytoma or paraganglioma. However, its sensitivity can be enhanced when combined with other imaging modalities like CT or MRI.

## Correct Answer Line
**Correct Answer: C. MIBG.**