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
Which of the following is diagnostic of the 'Rim sign' in a nephrogram? 
Select and explain. 
A. Pyelonephritis, B. Hyper nephroma, C. Polycystic kidney, D. Polycystic kidney
D: Polycystic kidney
Answer: D: Polycystic kidney 
Explanation: Ans. Polycystic kidney