This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrine, Subject name: Pathology
[{'content': 'Zellballen pattern is seen in? \nSelect and explain. \nA. Pheochromocytoma, B. Schwannoma, C. Acoustic neuroma, D. Transistional renal cell carcinoma', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. is \'a\' i.e., Pheochromocytoma * Pheochromocytoma is composed of polygonal to spindle shaped chromaffin cells or chief cells, clustered with sustentacular cells into small nests or alveoli (zellballen) by a rich vascular network. The nuclei are usually round to ovoid, with a stippled "salt & pepper" chromatin that is characteristic of most neuroendocrine tumors.* Zellballen pattern is also seen in paraganglioma.', 'role': 'assistant'}]