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
[{'content': 'Major cation in ECF \nSelect and explain. \nA. Na, B. K, C. PO4-2, D. Mg', 'role': 'user'}, {'content': "Answer: A \nExplanation: 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.)", 'role': 'assistant'}]