A 57-year old male presents with right arm pain of 4 weeks duration. He reports the pain began following a tennis match and has not improved with time. He describes the pain as an aching sensation that affects his lateral forearm that improves when he abducts the shoulder. He also describe a sensation of numbness in this right thumb. Reflex exam shows he has 1+ right biceps reflexes and 2+ right triceps reflexes which are both symmetric with the left side. Sensory exam shows paresthesias in the distribution of the right thumb. Motor exam shows no evidence of radial deviation with active wrist extension. Motor exam on the right shows 5/5 deltoid, 5/5 elbow flexion with the palms facing upward, 4/5 wrist extension, and 5/5 elbow extension, and 5/5 wrist flexion. What is the most likely etiology of his symptoms.