What is the most likely diagnosis for a patient with constant pelvic pressure, low back pain, and a firm, mobile, enlarged uterus?

The presentation of constant pelvic pressure, low back pain, and a firm, mobile, enlarged uterus aligns well with the diagnosis of uterine leiomyomas, commonly known as fibroids. These benign smooth muscle tumors of the uterus can often cause symptoms such as pelvic pressure, discomfort, and changes in menstrual patterns due to their size and location.

Uterine leiomyomas can vary significantly in size and may be located in different regions of the uterus (subserosal, intramural, or submucosal), which contributes to the symptomatology. The firm, mobile nature of the uterus on examination suggests the presence of fibroids rather than other conditions.

In the context of the other options, endometrial carcinoma typically presents with abnormal bleeding rather than constant pelvic pressure and would not generally manifest as a firm and mobile enlarged uterus. Uterine adenomyosis may present with similar pain and a bulky uterus but often leads to a softer consistency of the uterine tissue upon palpation. Endometrial polyps, while they can cause abnormal bleeding, do not typically cause pelvic pressure or an enlarged uterus noticeable on examination.

Thus, the clinical signs presented strongly support the diagnosis of uterine leiomyomas, making this the most likely diagnosis