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
Rim - sign on nephrogram is seen in 
Select and explain. 
A. Hydronephrosis, B. Absent kidney, C. Wilm's tumor, D. Renal cell carcinoma
A: Hydronephrosis
Answer: A: Hydronephrosis 
Explanation: Causes of Rim - nephrogram are :

Renal vein thrombosis.
Hydronephrosis.
Acute complete renal artery occlusion.