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
Prerenal and renal azotemia is differentiated on the basis of: 
Select and explain. 
A. Creatinine clearance, B. Serum creatinine level, C. Sodium fraction excretion, D. Urine bicarbonate level
C: Sodium fraction excretion
Answer: C: Sodium fraction excretion 
Explanation: Answer is C (Sodium Fraction Excretion):