A 27-year-old volleyball player complains of worsening right posterolateral shoulder pain and weakness for the past 4 weeks. She denies any injury to the shoulder. Her examination reveals 5/5 muscle strength with shoulder elevation, abduction and internal rotation. She is found to have weakness in external rotation with the elbow at the side and gross inspection is remarkable for mild atrophy along the posterior scapula. She has an unremarkable lift-off test. Which nerve and corresponding site of compression is most likely responsible?