A 67 year old male presents with a 1 year history of progressive lethargy, pruritis, and non-specific chest and abdominal discomfort that tends to come and go. He has a history of Hypertension and is on Amlodipine 5mg. He smokes 40 cigarettes a day and consumes up to 30 units of alcohol in a week.
On examination his blood pressure is 158/96, and he has a BMI of 35. You also notice he has a plethoric appearance.
Which one of the following options is the single most useful investigation in helping reach a diagnosis?