What is the most common cause of menorrhagia in a woman with an enlarged uterus?

The most common cause of menorrhagia in a woman with an enlarged uterus is fibroids. Uterine fibroids, also known as leiomyomas, are benign smooth muscle tumors of the uterus that can lead to heavy menstrual bleeding due to their location and size. Their growth can cause the uterus to enlarge, which directly contributes to the symptoms of menorrhagia. As fibroids increase the surface area of the endometrium (the lining of the uterus), they can cause prolonged or heavy menstrual periods.

In contrast, while hormonal imbalances can contribute to menstrual irregularities, they do not necessarily cause uterine enlargement specifically. Polycystic ovary syndrome (PCOS) is primarily associated with anovulation and may cause irregular bleeding, but it typically does not result in an enlarged uterus. Cervical cancer can cause abnormal bleeding, but it is less frequently a direct cause of heavy menstrual bleeding and is not usually associated with an enlarged uterus in the context of menorrhagia.

Therefore, among the options provided, fibroids are the most likely explanation for menorrhagia in a woman presenting with an enlarged uterus.