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
Excitable tissue at rest is least permeable to 
Select and explain. 
A. Na+, B. K+, C. Ca++, D. CI-
A: Na+
Answer: A: Na+ 
Explanation: Points to note
-        Concentration of ions

Na+ concentration higher in ECF 
K+ concentration higher in ICF 
Cl- concentration higher in ECF

-        K+ leak channels allow K+ to move from ICF to ECF

Na+ is prevented from entering the cell.

Thus diffusion is highest for K+, then Cl- and least for Na+, when the cell is at rest.