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': 'In congenital pyloric stenosis the defect usually lies in - antrum - \nSelect and explain. \nA. Nerve fibres, B. Circular muscle fibres, C. Longitudinal muscle fibres, D. Mucosa', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]