A 68-year-old man presented with a 3-day history of progressively weak legs, and is now unable to walk. There is no numbness or tingling, no speech or swallowing problems, no visual change and no weakness of the arms. He does not recall any symptoms like this before. He says he is usually fit and well, however has chronic back pain and has had problems with urinary dysfunction for which he has been taking tamsulosin. He also thinks he has lost weight. He is a lifelong smoker. What is the most likely underlying diagnosis?