This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
[{'content': 'Urinary retention in child is most commonly caused by - \nSelect and explain. \nA. Metal scab with ulceration, B. Post.urethral valve, C. Urethral stricture, D. Epispadias', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]