A 13-year-old girl began menstruation 1 year ago. She now has abnormal uterine bleeding, with menstrual periods that are 2 to 7 days long and 2 to 6 weeks apart. The amount of bleeding varies from minimal spotting to very heavy flow. On physical examination, there are no remarkable findings. A pelvic ultrasound scan shows no abnormalities. Which of the following is most likely to produce these findings?

Anovulatory cycles are a common cause of dysfunctional uterine bleeding in young women who are beginning menstruation and in women approaching menopause. There is prolonged estrogenic stimulation that is not followed by the secretion of progesterone. Ectopic pregnancy has acute findings and does not have a prolonged course. Endometrial carcinomas are rare in patients this age. Polyps are more common in older women. Submucosal leiomyomas are a cause of less variable bleeding and are more likely to be seen in older women.