AKT-2111

A 64-year-old man presents with left-sided shoulder pain. He has a medical history of hypertension, osteoarthritis, and COPD. During the examination, he shows good range of motion in the shoulder but experiences pain on the lateral aspect of the shoulder that radiates down to the upper arm and forearm. Additionally, he has some muscle wasting in his hand and a left-sided ptosis and miosis. What is the most suitable investigation to request next?