After a head injury, a patient has a left-sided torticollis. Which muscle is most likely responsible?

Torticollis, or "twisted neck," can occur after a head injury and is characterized by the abnormal positioning of the head, which can often be associated with muscle imbalance or spasms. In this case, the left-sided torticollis suggests that the head is tilted towards the left shoulder, with the chin pointing to the right.

The sternocleidomastoid muscle plays a significant role in the movement and positioning of the head. When there is abnormal contraction or spasm of the left sternocleidomastoid muscle, it will pull the head down and towards the left side, resulting in the characteristic tilt seen in torticollis. This muscle is responsible for flexing and rotating the head, and any injury or irritation affecting it can lead to a torticollis presentation on the same side.

Considering the options, the left sternocleidomastoid muscle's involvement in the twisting movement and its direct anatomical position makes it the most likely culprit for the left-sided torticollis observed in this patient after a head injury.