This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
Major cation in ECF 
Select and explain. 
A. Na, B. K, C. PO4-2, D. Mg
A: Na
Answer: A: Na 
Explanation: Na, K ATPase catalyzes the hydrolysis of ATP to adenosine diphosphate (ADP) and uses the energy to extrude three Na+ from the cell to ECF and take two K+ into the cell for each molecule of ATP hydrolyzed.(REF: GANONG'S REVIEW OF MEDICAL PHYSIOLOGY 23rd EDITION page no.47.)