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: Radiology
[{'content': 'Key hole sign is seen in \nSelect and explain. \nA. Hydronephrosis, B. Posterior urethral valve, C. Ectopic ureter, D. PCKD', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Key hole sign - Posterior urethral valve.\nCrescent sign - Hydronephrosis.', 'role': 'assistant'}]