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: Pediatrics
[{'content': 'Most common cause of hydronephrosis is children \nSelect and explain. \nA. PUJ obstruction, B. Ureterocele, C. Posterior urethral valve, D. Ectopic ureter', 'role': 'user'}, {'content': 'Answer: A \nExplanation: In children Most common cause of hydrophosis is Pelvi\xa0Ureteric junction obstruction.', 'role': 'assistant'}]