## **Core Concept**
Struma ovarii is a rare type of ovarian tumor that belongs to the category of sex cord-stromal tumors. It is characterized by the presence of thyroid tissue within the ovary. This tumor is significant because it can produce thyroid hormones, leading to clinical hyperthyroidism in some cases.

## **Why the Correct Answer is Right**
The correct answer, , indicates that Struma ovarii is composed of thyroid tissue. This tumor is essentially a teratoma that is composed entirely or predominantly of thyroid tissue. The thyroid tissue within Struma ovarii can be functionally active, producing thyroid hormones.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because Struma ovarii is not composed of mucinous epithelium; that would be more characteristic of a mucinous cystadenoma or cystadenocarcinoma of the ovary.
- **Option B:** This option is incorrect because, although some ovarian tumors can produce hormones, Struma ovarii is specifically known for being composed of thyroid tissue, not just being hormonally active.
- **Option D:** This option is incorrect because Struma ovarii is not composed of granulosa cells; that would be more characteristic of a granulosa cell tumor, another type of sex cord-stromal tumor.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Struma ovarii can cause hyperthyroidism due to the functional thyroid tissue within the tumor. It is also notable for sometimes being associated with ascites and, rarely, with pseudo-Meigs' syndrome (a condition characterized by ascites and pleural effusion that resolves upon tumor removal).

## **Correct Answer:** . thyroid tissue