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 prostate Ca. metastasis, lymph nodes most commonly affected are \nSelect and explain. \nA. Sacral LN, B. Obturator LN, C. Paraaortic LN, D. Iliac LN', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Obturator LN are first nodes involved in prostate Ca.', 'role': 'assistant'}]