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
Signs and symptoms of a Renal Failure first appear when Renal function deteriorates by more than: 
Select and explain. 
A. 20%, B. 40%, C. 60%, D. 80%
C: 60%
Answer: C: 60% 
Explanation: Answer is C (>60%): Signs of renal failure become apparent when renal function deteriorates by more than 60% ( residual renal function between 20-50%) while ove symptoms usually develop only after renal function has deteriorated by >75 - 80% (residual renal function <20%)