A young patient had a history of fall. The physician placed his hand on the patient's chin and instruct him to rotate his head to the opposite side against resistance. But he was unable to perform the action. Which of the nerve is injured in this patient?

The sternocleidomastoid muscle creates the borders for both the anterior and the posterior triangles of the neck, and is innervated by the spinal accessory nerve (CN XI). To test the sternocleidomastoid muscle, the physician will place her hand on the patient's chin and instruct him to rotate his head to the opposite side against resistance. If acting normally, the patient's muscle can be seen and palpated. The physician next will instruct the patient to shrug his shoulders against resistance. Patients with damage to CN XI will have diminished shoulder strength while shrugging on the injured side.