A 12 year old girl is brought in by her mother because she has been complaining about cyclic pelvic pain which occurs with every full moon. "She's been reading too much Twilight," the mother says. The girl denies any kind of abuse. She has not started menstruating yet. Abdominal exam shows mild suprapubic tenderness. External vaginal exam shows a vascular, bluish bulge protruding from the vaginal introitus while the patient is supine. Pelvic ultrasound is shown above; the white arrow indicates the vagina and the black arrow indicates the uterus.
Challenge: What's the diagnosis?
Image shown under Fair Use.