**Core Concept**
Medullary carcinoma of the thyroid is a type of neuroendocrine tumor that arises from the parafollicular cells (also known as C-cells) of the thyroid gland. This cancer is characterized by a specific histopathologic feature that distinguishes it from other types of thyroid cancers.

**Why the Correct Answer is Right**
Medullary carcinoma of the thyroid is characterized by the presence of amyloid deposits in the stroma. Amyloid deposits are abnormal collections of protein fibers that can be seen under the microscope. In medullary carcinoma, these amyloid deposits are composed of calcitonin, a hormone produced by the parafollicular cells. The presence of amyloid deposits is a key diagnostic feature of this cancer and can be seen in about 80-90% of cases.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Hürthle cell change is a feature of Hürthle cell carcinoma, a type of follicular carcinoma of the thyroid. While Hürthle cell change can be seen in other types of thyroid cancer, it is not a characteristic feature of medullary carcinoma.
**Option B:** This option is incorrect because papillary carcinoma of the thyroid is characterized by the presence of papillae and nuclear grooves, not amyloid deposits. While papillary carcinoma can have a variety of histopathologic features, amyloid deposits are not typically seen.
**Option C:** This option is incorrect because follicular carcinoma of the thyroid is characterized by the presence of capsular and/or vascular invasion, not amyloid deposits. While follicular carcinoma can have a variety of histopathologic features, amyloid deposits are not typically seen.

**Clinical Pearl / High-Yield Fact**
The presence of amyloid deposits in the stroma of a thyroid tumor is a key feature of medullary carcinoma and can be seen in about 80-90% of cases. This feature is often used to distinguish medullary carcinoma from other types of thyroid cancer.

**Correct Answer:** C. Amyloid deposits in the stroma.