Review Question - QID 106164

A 50-year-old man presents to his primary care physician's office because of pain in his upper right arm. He states that over the last few months, he has been having pain in his right arm that worsens with raising objects overhead. He also recently fell on his outstretched right hand, which seemed to worsen his pain. He has no prior medical problems and only takes a multivitamin. He has not had any prior surgeries. He does not smoke cigarettes and drinks alcohol socially. He denies illicit drug use. He is a construction worker. His temperature is 98.6°F (37.0°C), blood pressure is 120/70 mmHg, pulse is 70/min, and respirations are 16/min. On physical exam, he has no pain or weakness on internal or external rotation of the right arm. When his arm is internally rotated and the dorsum of his hand is placed on his sacrum, there is no pain when he lifts his hand off his back against resistance. When his arm is abducted to 90 degrees and externally rotated, he is only able to lower his arm with choppy movements. He is unable to abduct his arm from his side. Which of the following structures has he most likely injured?