A 27-year-old man had an intercostal catheter inserted through the fifth intercostal space in the midaxillary line after a crush injury to the chest. Two weeks after its removal he cannot raise his right arm above shoulder height. When he pushes against a wall, the medial border of the right scapula lifts away from the chest wall. Abduction of the shoulder to 90 degrees is full, adduction against resistance is strong, and sensation over the medial side of the upper arm is intact.
Injury to which one of the following nerves at the drain site best explains this man's scapular findings?