A 4-year-old boy is brought to the emergency department by his mother with a painful, swollen knee for the past two hours. She reports that the knee swelled after he fell on the carpet while running in the house. She witnessed the fall and reports that it was not a bad fall. He has no known medical history and is not taking any medications. Physical examination is consistent with a hemarthrosis in the affected knee. X-rays are negative for fracture.
Which of the following is the most likely underlying cause of the patient's predisposition to this type of injury?
- A) Factor VIII deficiency
- B) Factor IX deficiency
- C) Factor XI deficiency
- D) Immune thrombocytopenia
- E) Thrombotic thrombocytopenic purpura