MRCP2-4399

An 80-year-old woman presents to the rapid access chest clinic with a four-month history of progressive breathlessness, lethargy, anorexia, and a one stone weight loss. She is a housewife and smokes 15 cigarettes a day. Her husband, a retired plumber, recently passed away from a ‘chest problem’. The patient reports experiencing a dull right-sided chest pain for the last month, which is partially relieved with ‘low dose’ co-codamol prescribed by her GP. On examination, she appears dyspnoeic and cachectic. Examination of her chest reveals reduced vocal fremitus, percussion note, and breath sounds throughout the right lung. A chest x-ray shows a medium-sized right-sided pleural effusion, with thickening of the pleura in the right hemithorax. What investigation is most likely to lead to a diagnosis?