**Core Concept**
The patient's symptoms of dysmenorrhea and menorrhagia, combined with a symmetrically enlarged uterus, suggest a condition involving abnormal uterine bleeding and pelvic pain. This presentation is often associated with hormonal imbalances and anatomic changes in the uterus.

**Why the Correct Answer is Right**
The most likely diagnosis in this scenario is **Adenomyosis**. Adenomyosis is a condition where endometrial tissue grows into the myometrium, leading to uterine enlargement and symptoms of dysmenorrhea and menorrhagia. This condition is often associated with hormonal changes and can be exacerbated by pregnancy.

* The presence of a symmetrically enlarged uterus up to 14 weeks suggests a diffuse involvement of the myometrium, which is characteristic of adenomyosis.
* The symptoms of dysmenorrhea and menorrhagia are classic presentations of adenomyosis, which are thought to result from the abnormal growth of endometrial tissue and the subsequent hormonal imbalances.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Fibroids** may present with menorrhagia and dysmenorrhea, but they typically cause a uterus that is asymmetrically enlarged, with the fibroids often palpable as separate masses.
* **Option B:** **Endometrial hyperplasia** may cause menorrhagia, but it is less likely to cause a symmetrically enlarged uterus and is often associated with anovulatory cycles.
* **Option C:** **Uterine cancer** is a possibility in cases of menorrhagia and dysmenorrhea, but it is less likely in a 45-year-old woman with a symmetrically enlarged uterus and a history of multiparity.

**Clinical Pearl / High-Yield Fact**
Adenomyosis is often associated with a history of uterine surgery, such as cesarean sections or D&C procedures, which may have caused the endometrial tissue to grow into the myometrium.

**Correct Answer: C. Adenomyosis. Adenomyosis is a condition where endometrial tissue grows into the myometrium, leading to uterine enlargement and symptoms of dysmenorrhea and menorrhagia.**