A 35 year old woman complains of severe lower abdominal pain, which is worst during menstruation. Laparoscopic examination of the pelvis demonstrates multiple small brown spots on the surface of pelvic structures. Most of these lesions are cauterized, but biopsy of one of the remaining lesions reveals glandular tissue resembling normal endometrium. No cytologic atypia or abnormally shaped glands are seen. Which of the following is the most likely diagnosis?