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
Chronic Renal Failure is defined as Residual renal functional mass less than: 
Select and explain. 
A. 50%, B. 80%, C. 20%, D. 5%
C: 20%
Answer: C: 20% 
Explanation: Answer is C (20%): Function mass less than 20 percent of normal. Pathological Classification of Chronic Kidney Disease(CKD) Diminished Renal Reserve Renal Insufficiency Chronic Renal Failure End-Stage Renal Disease Residual Functional Renal Mass Residual GFR (% of Normal) 50 to 80 percent of Normal 20 to 50 percent of Normal Less than 20 percent of normal Less than 5 percent of normal Signs and Symptoms of Renal Failure are evident in the stage of Renal Insufficiency.