**Core Concept**
Sipple's syndrome, also known as Multiple Endocrine Neoplasia type 2A (MEN2A), is a rare genetic disorder characterized by the occurrence of medullary thyroid carcinoma, pheochromocytoma, and primary hyperparathyroidism. It is caused by a mutation in the RET proto-oncogene.

**Why the Correct Answer is Right**
Sipple's syndrome is a distinct clinical entity that is associated with a specific combination of endocrine tumors. The key features of Sipple's syndrome include medullary thyroid carcinoma, pheochromocytoma, and primary hyperparathyroidism. The correct answer is not included in this combination, making it the correct choice.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a feature of Sipple's syndrome. Medullary thyroid carcinoma is a key component of the syndrome, making this option incorrect.
**Option B:** Primary hyperparathyroidism is a feature of Sipple's syndrome, so this option is incorrect.
**Option C:** Pheochromocytoma is a feature of Sipple's syndrome, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Sipple's syndrome is a distinct clinical entity with a specific combination of endocrine tumors. The presence of medullary thyroid carcinoma, pheochromocytoma, and primary hyperparathyroidism should raise suspicion for Sipple's syndrome.

**Correct Answer: C. Pheochromocytoma is a feature of MEN2B, not MEN2A (Sipple's syndrome).**