A 17 year old female presents with a 6 month history of amenorrhoea. She denies ever being sexually active and prior to this she had regular periods once a month and had no intermenstrual bleeding. She has no past medical history and is not on any regular medication. She has a BMI of 22. Abdominal examination is unremarkable and due to the fact that she has never been sexually active a pelvic examination was not offered.

What is the next most appropriate investigation?