**Core Concept**
The Zellballen pattern is a characteristic histopathological feature observed in certain neuroendocrine tumors, particularly in pheochromocytomas and paragangliomas. This pattern is characterized by clusters of tumor cells (Zellballen) separated by a loose network of sustentacular cells.

**Why the Correct Answer is Right**
The Zellballen pattern is a result of the unique cellular arrangement in these tumors. The tumor cells are organized into clusters or nests, which are surrounded by sustentacular cells that provide structural support and play a role in the secretion of catecholamines. The sustentacular cells are typically positive for S100 protein, which is a useful marker for identifying these cells. The Zellballen pattern is a key diagnostic feature of pheochromocytomas and paragangliomas, and it helps distinguish these tumors from other types of neuroendocrine tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Zellballen pattern is not characteristic of medullary thyroid carcinoma, which typically shows a solid or trabecular pattern.
**Option B:** This option is incorrect because the Zellballen pattern is not seen in carcinoid tumors, which typically show a uniform arrangement of tumor cells.
**Option C:** This option is incorrect because the Zellballen pattern is not characteristic of adrenal cortical tumors, which typically show a diffuse or nodular pattern.

**Clinical Pearl / High-Yield Fact**
The Zellballen pattern is a key diagnostic feature of pheochromocytomas and paragangliomas, and it should be considered in the differential diagnosis of any neuroendocrine tumor that shows a characteristic clustering of tumor cells.

**Correct Answer: D. Pheochromocytoma/Paraganglioma**