A 43-Year-Old Woman, Gravida 3 Para 3, Comes to the Office

A 43-year-old woman, gravida 3 para 3, comes to the office due to heavy and painful menstrual bleeding over the past 4 months.  The patient's last menstrual period was 3 weeks ago.  Menarche was at age 11, and menstrual periods last for 3-5 days and occur every 30 days.  She is sexually active with her husband and does not have pain with intercourse.  The patient had a bilateral tubal ligation 3 years ago after the birth of her last child.  She takes no medications and has no allergies.  BMI is 24 kg/m².  Vital signs are normal.  On bimanual examination, the uterus is uniformly enlarged and tender.  Urine beta-hCG is negative.  Which of the following is the most likely cause of this patient's symptoms?

A) Benign myometrial smooth muscle cell proliferation
B) Blastocyst implantation in the fallopian tube
C) Endometrial glands and stroma within the myometrium
D) Localized overgrowth of endometrium into the uterine cavity
E) Unregulated endometrial proliferation with increased gland-to-stroma ratio