A 65-year-old male comes to the OPD complaining of ongoing double vision which is worse when he is watching TV in the evening. He has also noticed that he feels weaker towards the end of the day and that his face is slightly puffy. On inspection, there is a paial ptosis of the left eye lid and his mouth hangs slightly open at rest. On examination, there are visibly distended neck veins. What is the most likely diagnosis?