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': "Rim - sign on nephrogram is seen in \nSelect and explain. \nA. Hydronephrosis, B. Absent kidney, C. Wilm's tumor, D. Renal cell carcinoma", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Causes of Rim - nephrogram are :\n\nRenal vein thrombosis.\nHydronephrosis.\nAcute complete renal artery occlusion.', 'role': 'assistant'}]