A 40 year old man presents with episodes of syncope. He has a sudden loss of consciousness which is short-lived and he quickly returns to normal. He can sometimes feel lightheaded beforehand, but he is otherwise fit and well. These episodes have only occurred in the bathroom after passing urine, and usually following an evening when he has had alcohol. His wife found him on one occasion and says there was no limb shaking or tongue biting.
What is the most likely diagnosis?