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You are seeing a 13 year old girl for menorrhagia. This is her second menses ever, and she has been having to change her pad every 2 hours for 7 days. Her physical exam is unremarkable except for some mild tachycardia and ongoing vaginal bleeding. She has not had any surgeries, but she does recall a nosebleed episode that lasted 15 minutes, and she does endorse easy bruising. She is not taking any medications and has no significant past medical history. There is no known family history of a bleeding disorder, and she has one older brother who is healthy. 

Which laboratory test do you most expect to be abnormal?