A 24-year-old man returns to clinic three weeks after coming off his motorbike onto his right shoulder. Radiographs at the time showed a fracture through the surgical neck of the humerus, treated in a sling. He still cannot lift the arm from his side, and the shoulder has lost its rounded contour. Sensation is reduced over the outer aspect of the upper arm. Elbow flexion, elbow extension, wrist extension and grip are full.
Injury to which one of the following nerves best explains this man's shoulder findings?