## **Core Concept**
The question tests knowledge of ovarian tumors, specifically those that can contain teeth. This presentation is classic for a type of germ cell tumor. Germ cell tumors arise from the cells that give rise to the ova and can contain various tissues, including hair, muscle, and teeth.

## **Why the Correct Answer is Right**
The correct answer, **D. Dermoid cyst (Mature teratoma)**, is the most likely diagnosis. Dermoid cysts, or mature teratomas, are a type of germ cell tumor that can contain well-differentiated tissues from all three germ layers, including ectoderm, mesoderm, and endoderm. The presence of a well-formed tooth on plain x-ray is highly suggestive of a dermoid cyst, as these tumors can contain teeth, hair, and sometimes even bone or cartilage. This condition is common in young women and can present as a lower abdominal mass.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although not specified, common ovarian tumors like serous or mucinous cystadenomas do not typically contain teeth.
- **Option B:** This option is incorrect as there's no common ovarian tumor directly associated with the given description that would fit better than dermoid cyst.
- **Option C:** This option is incorrect because, while immature teratomas (also a type of germ cell tumor) can contain a variety of tissues, they are less likely to have well-formed structures like teeth and are more commonly associated with a worse prognosis due to their potential for malignancy.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that dermoid cysts (mature teratomas) can sometimes cause ovarian torsion, especially if they grow large. This condition is a surgical emergency. Also, while they can contain teeth and other structures, the presence of these elements helps in the diagnosis, often made with imaging.

## **Correct Answer:** D. Dermoid cyst (Mature teratoma)