Assignment task: A 17-year-old boy is brought to his physician by his mother for the evaluation of increased bruising for 3 weeks. The mother reports that the patient has also had two episodes of nose bleeding in the last week that subsided spontaneously within a few minutes. The boy was born at term and has been healthy except for an episode of gastroenteritis 5 weeks ago that resolved without treatment. The patient is at the 48th percentile for height and 43rd percentile for weight. He appears healthy and well nourished. His temperature is 36.5°C (97.7°F) pulse is 100/min and his blood pressure is 100/65 mm Hg. There are a few scattered petechiae over the trunk and back. The remainder of the examination shows no abnormalities. Laboratory studies show.