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
[{'content': 'Which of the following is the most abundant extracellular ion? \nSelect and explain. \nA. Calcium, B. Potassium, C. Sodium, D. Chloride', 'role': 'user'}, {'content': "Answer: C \nExplanation: Sodium is the most abundant extracellular ion, whereas potassium is the most abundant intracellular ion. Phosphate is the most abundant intracellular anion and chloride is the most abundant extracellular anion. Ref: Human anatomy and physiology By Elaine N. Marieb, Katja Hoehn, 2010, Page 625; Guyton's physiology 11th edition page 46, 358.", 'role': 'assistant'}]