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 kidney disease is defined as GFR less than (mL/ min/ 1.73 m2): March 2004 
Select and explain. 
A. 60, B. 70, C. 80, D. 90
A: 60
Answer: A: 60 
Explanation: Ans. A i.e. 60