MRCP2-3980

A 55-year-old man presents to his GP with complaints of weight loss and increased frequency of stool over the past four months. He had a severe case of influenza one month ago and has been feeling more stressed at work. He also reports ankle swelling and frothy urine. On examination, he has evidence of pitting edema and a 2 cm firm non-tender liver edge. Investigations reveal low hemoglobin, low MCV, elevated ESR, and elevated bilirubin and alkaline phosphatase. CXR shows bilateral pleural effusions. What is the most important investigation to guide further management?