## **Core Concept**
Struma ovarii is a rare type of ovarian tumor that is characterized by the presence of thyroid tissue within the ovary. It is a form of mature teratoma that primarily consists of thyroid tissue. This condition can lead to hyperthyroidism due to the functional thyroid tissue.

## **Why the Correct Answer is Right**
Struma ovarii is indeed a type of tumor that contains **ectopic thyroid tissue**. This tumor is considered a mature teratoma, which is a type of germ cell tumor. The presence of thyroid tissue within the ovary can lead to symptoms of hyperthyroidism because the thyroid tissue can produce thyroid hormones. The majority of struma ovarii cases are **benign**. Therefore, the statement that it is included in teratoma and is benign supports the correct classification of struma ovarii.

## **Why Each Wrong Option is Incorrect**
- **Option B (Ectopic ovarian tissue):** This option is incorrect because struma ovarii refers specifically to the presence of thyroid tissue within the ovary, not ovarian tissue in an ectopic location.
- There are no details provided for options A, C, and D to assess their incorrectness directly, but based on the correct answer and explanation, we can infer which ones are incorrect.

## **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 essential to consider struma ovarii in the differential diagnosis of ovarian masses and hyperthyroidism. The condition often presents with symptoms of thyrotoxicosis and is usually diagnosed post-operatively after histopathological examination.

## **Correct Answer:** D.