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': 'Lactating women with painful breast, 1st investigation to be done should be: \nSelect and explain. \nA. USG, B. Mammography, C. CT, D. MR1', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. USG', 'role': 'assistant'}]