Solution: The lower two thirds of the vagina develop from the urogenital sinus, while the the upper vagina, cervix, uterus, fallopian tubes, and ovaries develop from the Müllerian duct system. Failure of normal fusion or canalization during development can result in cervical stenosis or atresia, vaginal atresia, or transverse vaginal septa (hematometra). In addition, an imperforate hymen (hematocolpos or hematometrocolpos) can also occur and is in fact the most common etiology for outflow obstruction. In this case, you can see a normal appearing uterus proximally, but the vagina is severely dilated with blood products consistent with hematocolpos and by far the most likely etiology is an imperforate hymen, which can be surgically corrected (hymenotomy). Aspiration should not be performed prior to surgical correction because of the risk of infection.