Review Question - QID 108914

A 6-year-old boy presents to the emergency department after falling from his scooter. He has dull, aching pain along his left side where he hit the ground. He fell sideways off the scooter as he rounded a curve in the road. He has never had any serious injuries but that he always seems to bruise easily, especially after he started playing soccer this fall. His parents deny that he has an abnormal number of nosebleeds or bleeding from the gums. They have never seen blood in his stool or urine. His mother notes that her brother has had similar problems. His temperature is 98.6°F (37°C), blood pressure is 112/74 mmHg, pulse is 82/min, and respirations are 11/min. On physical exam, the patient has extensive bruising of the lateral left thigh and tenderness to palpation. Laboratory tests are performed and reveal the following:

Hemoglobin: 14 g/dL
Hematocrit: 41%
Mean corpuscular volume: 89 µm3
Reticulocyte count: 0.8%
Leukocyte count: 4,700/mm3
Prothrombin time (PT): 13 seconds
Partial thromboplastin time (PTT): 56 seconds
Bleeding time (BT): 4 minutes

Which of the following is the most likely underlying pathophysiology?