A 38-year-old woman is seen by a gynecologist on a routine visit. A complete history is taken and is non-contributory except for the fact that the patient's menstrual irregularities apparently developed insidiously over the preceding decade. Screening physical examination is performed. Routine serum chemistries and complete blood count are sent, as are Pap smear studies. Pelvic ultrasound studies are also performed. Pregnancy test is negative. Endometrial biopsy shows proliferative endometrium. All other results are within normal limits, except for evidence of a mild iron deficiency anemia. Which of the following is the most likely diagnosis?