Review Question - QID 212809

A 16-year-old boy is brought to the emergency department after being tackled at a football game. Per his mom, he is the quarterback of his team and was head-butted in the left shoulder region by the opposing team. Shortly after, the mother noticed that his left arm was hanging by his torso and his hand was “bent backwards and facing the sky.” The patient denies head trauma, loss of consciousness, sensory changes, or gross bleeding. A physical examination demonstrates weakness in abduction, lateral rotation, flexion, and supination of the left arm and tenderness of the left shoulder region with moderate bruising. Radiograph of the left shoulder and arm is unremarkable. Which of the following is most likely damaged in this patient?