USMLE Question #492: Real Exam Question with Answer & Explanation

A 20-year-old woman comes to the physician because of a 5-year history of heavy bleeding with menses that often requires her to change her sanitary pads three times hourly. Menses occur at regular 28-day intervals. She recently sustained a minor cut to her finger, and the bleeding took longer to stop than usual. She has not had easy bruising or change in weight. She only takes an oral contraceptive, but she has not been sexually active for the past 6months. Her temperature is 37.5°C (99.5°F), pulse is 72/min, respirations are 12/min, and blood pressure is 120/66 mmHg. Physical examination shows mildly pale conjunctivae. Pelvic examination shows no abnormalities. Laboratory studies show: A Pap smear shows no abnormalities. Which of the following hematologic disorders is the most likely cause of this patient's menorrhagia?