Review Question - QID 4231

A 52-year-old male presents with a chief complaint of left arm pain following a tennis match. He reports the symptoms began 3 weeks ago, and have been present ever since. He describes diffuse, moderate pain that radiates from the base of his neck to his forearm. The pain is relieved by elevating his left arm. He also describes numbness and tingling in his index, long, and ring fingers. He denies any numbness or tingling in his thumb. On physical exam he is noted to have decreased triceps strength on the affected side, and a decreased triceps reflex. These symptoms are most likely caused by: