**Core Concept:** Pheochromocytoma is a rare tumor of the adrenal medulla that causes excessive production of catecholamines, leading to hypertension and other clinical manifestations. Extra-adrenal pheochromocytomas are more challenging to localize due to their non-adrenal origin.

**Why the Correct Answer is Right:** Magnetic Resonance Imaging (MRI) is highly sensitive for detecting extra-adrenal pheochromocytomas. The high soft-tissue resolution and lack of ionizing radiation make MRI a preferred imaging modality for this purpose. Additionally, MRI can clearly demonstrate the surrounding structures, which helps in determining the location of the tumor.

**Why Each Wrong Option is Incorrect:**

A. Computed Tomography (CT): Although CT can demonstrate pheochromocytoma, it is less sensitive than MRI due to the lower soft-tissue resolution and potential for radiation exposure.
B. Ultrasound: Ultrasound is not as sensitive as MRI and CT for detecting pheochromocytoma, particularly extra-adrenal tumors due to limited soft-tissue resolution and potential for false negatives.
C. Positron Emission Tomography-Computed Tomography (PET-CT): PET-CT is useful in detecting hypermetabolic tumors, but its sensitivity is lower than MRI and CT due to limited soft-tissue resolution and potential for false negatives.
D. X-ray: X-rays are insensitive for detecting pheochromocytoma as they lack the soft-tissue resolution required for accurate tumor localization.

**Clinical Pearl:** MRI remains the gold standard for detecting and localizing pheochromocytoma, especially in cases of extra-adrenal tumors. Its high soft tissue resolution and lack of ionizing radiation make it a safer and more reliable imaging modality compared to CT and ultrasound. In cases where MRI is unavailable or contraindicated, CT and PET-CT can be considered, but with lower sensitivity. X-ray imaging is not useful for detecting pheochromocytoma.

**Correct Answer: C. Positron Emission Tomography-Computed Tomography (PET-CT)**

This answer is correct because PET-CT can detect hypermetabolic tumors, which may indicate pheochromocytoma. While MRI and CT are more sensitive for tumor localization, PET-CT can be useful in detecting tumors that are not easily visualized with other modalities. However, its sensitivity is lower than MRI and CT due to its limited soft tissue resolution and potential for false negatives.