Review Question - QID 109643

An 11-month-old male is brought to the emergency room by his mother. The mother reports that the child is in severe pain and has not moved his right leg since earlier this morning when he was crawling on the floor. The child did not fall or sustain any obvious injury. The child’s past medical history is notable for anemia and recurrent infections since birth. His temperature is 99.1°F (37.3°C), blood pressure is 100/65 mmHg, pulse is 120/min, and respirations are 22/min. Physical examination reveals mild macrocephaly and hepatosplenomegaly. Palpation of the right femur seems to exacerbate the child’s pain. A radiograph demonstrates a transverse mid-shaft femur fracture. Which of the following is the most likely cause of this patient’s condition?