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: Medicine
Azotemia occurs when - 
Select and explain. 
A. GFR 50% of normal, B. GFR 80% of normal, C. GFR 20% - 50% of normal, D. GFR 25% of normal
C: GFR 20% - 50% of normal
Answer: C: GFR 20% - 50% of normal 
Explanation: None