This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'Which of the following is most reliable feature of malignant transformation of pheochromocytoma ? \nSelect and explain. \nA. Presence of mitotic figures, B. Capsular invasion, C. Vascular invasion, D. None', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. is None "Because benign and malignant pheochromocytomas may have an identical histological appearance, the only absolute criterion of malignancy is metastasis." "The diagnosis of malignant pheochromocytoma is made only when metastasis is demonstrated."', 'role': 'assistant'}]