A 17-year-old girl is brought to the physician for a routine physical. She has no past medical history. Physical examination shows a cut on her left index finger. She remarks that last week she cut herself and it took awhile to stop bleeding. Menses occur every 28 days, last 5 days, and produce clots. Laboratory studies are shown.
Hemoglobin 13 g/dL
Platelets 200,000/μL
Prothrombin time 12 seconds
Partial thromboplastin time 34 seconds
Which of the following is most likely impaired in this patient?
