A 75-year-old man presents with dysphagia and chest pain that have been progressively worsening for the past 4 months despite a trial of proton pump inhibitors. He denies any weight loss or anorexia. During examination, you observe a partial ptosis on the right side and the patient reports double vision during eye movement assessment. Sustained upward gaze worsens the ptosis. A chest x-ray is ordered:
What is the probable diagnosis?