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
The most specific marker of renal function is - 
Select and explain. 
A. Creatinine clearance, B. Insulin clearance, C. Blood area, D. Serum creatinine
D: Serum creatinine
Answer: D: Serum creatinine 
Explanation: None