This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Body fluid compaments, Subject name: Physiology
Which substances are best suited to measure interstitial fluid volume? 
Select and explain. 
A. Inulin and heavy water, B. Inulin and 22Na, C. Heavy water and 125I-albumin, D. Inulin and 125I-albumin
D: Inulin and 125I-albumin
Answer: D: Inulin and 125I-albumin 
Explanation: Interstitial fluid volume= ECF volume - Plasma volume Inulin best indicator of ECF volume and 125I-albumin is most commonly used for plasma volume estimation