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: Biochemistry
Kidney plays a role in maintaining constancy of milieu interior. Milieu interior refers to: 
Select and explain. 
A. ECF, B. ICF, C. Plasma, D. Serum
A: ECF
Answer: A: ECF 
Explanation: Milieu interior refers to the extracellular environment. The main function of kidney is homeostasis, the maintenance of the constancy of the milieu interior. Ref: Fundamentals of Medical Physiology by Joel Michael, Sabyasachi Sircar, 2010, Chapter 53.