<prompt> A young patient had a history of fall. The physician placed his hand on the patients 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?Posterior auricular nerve10th cranial nerveSpinal accessory nerve12th cranial nerve  <response>: 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 patients chin and instruct him to rotate his head to the opposite side against resistance. If acting normally, the patients 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. 