An 18-year-old female passenger injured in a rollover car crash was rushed to the emergency department. After the patient is stabilized she undergoes physical examination. She demonstrates considerable weakness in her ability to flex her neck, associated with injury to CN XI. Which of the following muscles is most probably affected by nerve trauma?

The iliocostalis thoracis muscle is found in the deep back and functions to maintain posture. It is not associated with neck flexion. The sternocleidomastoid muscle is innervated by CNXI and functions in contralateral rotation and bilateral flexion of the neck. Rhomboid major and minor are both innervated by the dorsal scapular nerve and serve to adduct the scapulae. Teres major is innervated by the lower subscapular nerve and serves to medially rotate and adduct the humerus.