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?

This benign condition is endometriosis, rather than metastatic endometrial cancer. Endometriosis is defined as a benign growth of endometrium at sites at which it does not normally occur (excluding the myometrium, at which site it is called adenomyosis). Endometriosis is common and is a significant cause of both pain and pelvic scarring. Most of the problems occur because the abnormally located endometrium responds to hormonal control and may menstruate, producing a very irritating fluid. Endometriosis may apparently be staed either by seeding of menstrual cells in the pelvis (entering through the open end of the fallopian tubes) or by metaplasia of mesothelial or other cells (accounting for rare, well-documented cases of endometriosis of bizarre sites such as nasal mucosa or lungs). Acute endometritis is acute inflammation of the endometrium. Adenomyosis is characterized by foci of endometrium deep in the myometrium. Chronic endometritis is chronic inflammation of the endometrium. Ref: Hoffman B.L., Schorge J.O., Schaffer J.I., Halvorson L.M., Bradshaw K.D., Cunningham F.G., Calver L.E. (2012). Chapter 10. Endometriosis. In B.L. Hoffman, J.O. Schorge, J.I. Schaffer, L.M. Halvorson, K.D. Bradshaw, F.G. Cunningham, L.E. Calver (Eds), Williams Gynecology, 2e.