**Core Concept**
Pheochromocytomas are rare catecholamine-secreting tumors originating from the chromaffin cells of the adrenal medulla or sympathetic nervous system. The distinction between benign and malignant pheochromocytomas is crucial for determining prognosis and treatment. Histopathological examination is essential for this purpose.

**Why the Correct Answer is Right**
Malignancy in pheochromocytomas is typically determined by the presence of metastasis or invasion into adjacent tissues. One of the key histopathological criteria for malignancy is the presence of **vascular invasion**, which indicates the tumor's ability to invade and spread through blood vessels. This is often assessed on intraoperative frozen sections by the pathology laboratory. Vascular invasion is a reliable indicator of malignancy in pheochromocytomas.

**Why Each Wrong Option is Incorrect**
**Option A:** Mitotic rate is not a reliable criterion for differentiating between benign and malignant pheochromocytomas. While high mitotic rates may suggest malignancy, they can also be seen in benign tumors.

**Option B:** Capsular invasion is a feature that can be seen in both benign and malignant pheochromocytomas. It is not a reliable criterion for distinguishing between the two.

**Option C:** Nuclear pleomorphism is a feature that can be seen in both benign and malignant pheochromocytomas. While it may suggest malignancy, it is not a reliable criterion for distinguishing between the two.

**Clinical Pearl / High-Yield Fact**
When examining intraoperative frozen sections from a pheochromocytoma, the pathologist should focus on identifying vascular invasion, as this is a reliable indicator of malignancy. This is particularly important in cases where the patient has a history of recurrent or metastatic disease.

**Correct Answer: C. Capsular invasion is a feature that can be seen in both benign and malignant pheochromocytomas. While it may suggest malignancy, it is not a reliable criterion for distinguishing between the two.