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': 'Investigation of choice for hydronephrosis – \nSelect and explain. \nA. Excretory Urography, B. USG, C. MRI, D. Cystourethrography', 'role': 'user'}, {'content': 'Answer: B \nExplanation: "Ultrasonography is the initial modality of choice in the investigation of hydronephrosis".\xa0—Imaging for students.', 'role': 'assistant'}]