A 16-year-old girl presents with urinary retention and a history of crampy abdominal pain. What is the most likely diagnosis?

The presentation of a 16-year-old girl with urinary retention and crampy abdominal pain aligns well with the diagnosis of haematocolpos. Haematocolpos occurs when menstrual blood accumulates in the vagina due to an obstruction, which can be caused by a hymenal membrane that hasn’t fully opened or other anatomical causes. The accumulation of blood can lead to significant abdominal pain and discomfort, as well as urinary retention due to pressure effects on the bladder and urinary tract.

In a case of haematocolpos, the signs and symptoms often present as abdominal pain along with changes in urinary function because the retained blood can put pressure on surrounding structures, including the bladder. This situation is particularly relevant in adolescent females who are newly menstruating and may experience anatomical variations that predispose to obstruction.

Other conditions listed, such as pelvic inflammatory disease, ovarian cysts, and endometriosis, may also manifest with abdominal pain but do not typically present with urinary retention stemming directly from an accumulation of menstrual blood. Therefore, haematocolpos emerges as the most fitting diagnosis given the specific symptoms described.