Shaken baby syndrome

A 6-month-old boy is brought into the emergency department by his parent following a motor vehicle accident. The parent states they were stopped at an intersection when another car hit hers in the rear. The patient was restrained in a car seat during the accident. At arrival, the patient’s temperature is 37.6°C (99.7°F), blood pressure is 95/47 mmHg, and pulse is 109/min. Weight is at the 10th percentile and height is at the 60th percentile. The patient is inconsolable by his parent. Physical examination reveals several bruises over the trunk and extremities at different stages of healing, which the parent attributes to falls that occurred while he was learning to walk. There is significant tenderness over the right leg and any manipulation of the extremity causes pain. Imaging reveals a spiral fracture of the femur. Which of the following is the most likely cause of this patient’s symptoms?