This case demonstrates hydro/hematocolpos. The mass at the superior end of the cyst is the cavity of the uterus. If pressure in the system is high the uterine cavity may be opened out. The pressure here is not high and there is a suggestion that the lower end of the cyst actually has a small leak as shown by signal arising just below the tapered end.

Final diagnosis: Congenital hydrocolpos due to marked stenosis of the introitus.