Winged scapula

A 24-year-old man comes to the office because of right shoulder pain and instability for a week. He states that he first noticed the pain while he was weight-lifting with his friends at the gym. Since the pain began, he has been unable to lift heavy weights above his head using his right arm. He also finds that he experiences shoulder pain, particularly while sitting on chairs. Examination of the shoulder is pictured. Which of the following nerves has most likely been injured?