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: Physiology
The most sensitive index for renal tubular function is 
Select and explain. 
A. Specific gravity of urine, B. Blood urea, C. GFR, D. Creatinine clearance
A: Specific gravity of urine
Answer: A: Specific gravity of urine 
Explanation: The main function of renal tubules is the concentration of urine and this can be measured by the specific gravity of urine.