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': 'Preferred method for obtaining dense nephrograms \nSelect and explain. \nA. Increased concentration of contrast, B. Dehydration, C. Uretric compression, D. Rapid injection of contrast', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Rapid (bolus) injection of contrast is preferred method.', 'role': 'assistant'}]