A palpable mass is noted in the right lobe of the thyroid of a 45-year-old man who visits his physician for a periodic checkup. A biopsy is performed and results in a diagnosis of medullary carcinoma of the thyroid. Which of the following histologic features of thyroid disease would most likely be present in this biopsy specimen?

## **Core Concept**
Medullary thyroid carcinoma (MTC) is a type of thyroid cancer that originates from the parafollicular cells, also known as C cells, which produce **calcitonin**. This cancer is distinct from other thyroid cancers that arise from follicular epithelial cells. The histological features of MTC are unique and help in its diagnosis.

## **Why the Correct Answer is Right**
The correct answer, , shows **amyloid deposits** in the stroma, which is a characteristic feature of medullary thyroid carcinoma. MTC is known for its amyloid-rich stroma, which is derived from the precalcitonin peptide. The presence of amyloid deposits, along with the tumor cells' characteristic spindle or polygonal shape and their tendency to form sheets or nests (sometimes with a spindle cell pattern), is diagnostic of MTC. The calcitonin produced by these cells can often be demonstrated by immunohistochemical staining, further supporting the diagnosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option does not describe a characteristic feature specific to medullary thyroid carcinoma.
- **Option B:** This option might represent a feature seen in other thyroid conditions but is not specific or diagnostic for MTC.
- **Option C:** While certain types of thyroid cancers have specific histological features like papillary structures or follicular formation, these are not characteristic of MTC.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients with medullary thyroid carcinoma may have elevated levels of **calcitonin**, which can serve as a tumor marker for diagnosis and follow-up. Additionally, MTC can be part of multiple endocrine neoplasia (MEN) syndromes, specifically MEN 2A and MEN 2B, which also involve pheochromocytoma and hyperparathyroidism.

## **Correct Answer: .**

✓ Correct Answer: C. Tumor cells embedded in an amyloid-laden stroma