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 appears when residual GFR is: 
Select and explain. 
A. 50% -80 % of normal, B. 20-50% of normal, C. 10 -20% of normal, D. < 5% of normal
B: 20-50% of normal
Answer: B: 20-50% of normal 
Explanation: Answer is B (20-50% of Normal): Azotemia appears in the stage of Renal Insufficiency when the residual renal function (Residual GFR) is between 20 to 50 percent of normal.