The parents ofa 13-month-old boy are contacted by a local social agency because the day care repoed them for suspected child abuse based on "too may bruises." The parents deny the abuse and promptly arragne to have their child seen by a medical specialist. The child has multiple large bruises on his legs, anus, and buttocks. No skin abrasions and no "pattern marks" are seen. PTT is prolonged and PT and bleeding time are normal. Which of the following is the most probable