This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General, Subject name: Physiology
The major ion in extracellular compartment 
Select and explain. 
A. K+, B. Na+, C. Cl-, D. HCO3-
B: Na+
Answer: B: Na+ 
Explanation: Ans: b (Na+) Ref: Ganong 22nd ed/ p. 30, 32Most abundant ion in ECF is Na+.ECFMost abundant ion - Na+Most abundant cation - Na+Most abundant anion - Cl-ICF:-Most abundant cation - K+Most abundant anion - PO43- (followed by protein).ECF contains more of Na+, Cl-, HCO3, Ca2+.ICF contains more of K+, PO4, organic anion, protein, magnesium.