**Core Concept**
Pheochromocytomas are rare, catecholamine-secreting tumors arising from chromaffin cells in the adrenal medulla or sympathetic nervous system. Malignant transformation is a concern in these tumors, but it can be challenging to diagnose. The key to identifying malignant transformation lies in understanding the clinical and pathological features that distinguish it from benign pheochromocytomas.

**Why the Correct Answer is Right**
The most reliable feature of malignant transformation of pheochromocytoma is the presence of distant metastases, particularly to the liver, lungs, or bones. This is because malignant pheochromocytomas have the ability to invade surrounding tissues and spread to distant sites through the bloodstream. The presence of metastases is a hallmark of malignancy and is associated with a poorer prognosis. In contrast, benign pheochromocytomas are typically confined to the adrenal gland or surrounding tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** Nuclear DNA content is not a reliable feature of malignant transformation of pheochromocytoma. While changes in DNA content can occur in malignant tumors, they are not specific to pheochromocytomas and can be seen in both benign and malignant tumors.

**Option B:** Histological features such as mitotic rate and cellular pleomorphism can be seen in both benign and malignant pheochromocytomas and are not reliable indicators of malignancy.

**Option C:** Tumor size is not a reliable feature of malignant transformation of pheochromocytoma. Large tumors can be benign, and small tumors can be malignant.

**Clinical Pearl / High-Yield Fact**
The presence of distant metastases is the most reliable feature of malignant transformation of pheochromocytoma. It is essential to consider the possibility of malignancy in patients with pheochromocytomas, particularly those with symptoms of metastatic disease or those who have undergone previous surgery for a pheochromocytoma.

**Correct Answer:** C. Distant metastases.