**Core Concept**
Menometrorrhagia, or irregular and heavy menstrual bleeding, can be caused by various gynecological conditions, including ovarian tumors. The presence of a solid adnexal mass and hyperplastic endometrium without cellular atypia on biopsy suggests a hormonal cause for the menometrorrhagia.

**Why the Correct Answer is Right**
The most likely lesion underlying her menstrual abnormalities is an ovarian tumor that secretes estrogen, leading to endometrial hyperplasia. The 7-cm solid right adnexal mass is likely an ovarian tumor, and the presence of hyperplastic endometrium without cellular atypia on biopsy suggests that the tumor is producing estrogen. The endometrial biopsy is crucial in ruling out endometrial cancer, which would have shown cellular atypia.

*Option A:* A dermoid cyst (mature teratoma) is unlikely to cause menometrorrhagia due to estrogen production, as it is a benign germ cell tumor that does not typically secrete hormones.

*Option B:* A mucinous cystadenoma is a type of epithelial ovarian tumor, but it is not typically associated with hormonal production or menometrorrhagia.

*Option C:* A Brenner tumor is a type of epithelial ovarian tumor, but it is not typically associated with hormonal production or menometrorrhagia.

*Option D:* A granulosa cell tumor is a type of sex cord-stromal tumor that can produce estrogen, leading to menometrorrhagia and endometrial hyperplasia. This is the most likely diagnosis given the clinical presentation and biopsy results.

**Clinical Pearl / High-Yield Fact**
A granulosa cell tumor can produce estrogen, leading to menometrorrhagia and endometrial hyperplasia. It's essential to consider this diagnosis in a woman with irregular and heavy menstrual bleeding and a solid adnexal mass on ultrasound.

**Correct Answer:** D. A granulosa cell tumor is the most likely lesion underlying her menstrual abnormalities.