This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Membrane potiential, Subject name: Physiology
The ion that contributes maximally to the development of RMP is: 
Select and explain. 
A. Na+, B. K+, C. Cl-, D. Ca++
B: K+
Answer: B: K+ 
Explanation: A resting cell is much more permeable to K+ as compared to Na+ or other ions. During resting conditions, if all the ions are moving down their concentration gradients to finally reach equilibrium, the movement of K+ will be much greater. Thus, K+ diffusing out, down its concentration gradient will be the greatest contributor to the RMP. As K+ diffuses outward to reach equilibrium, it drags the membrane potential to its own equilibrium potential. (Equilibrium potential for K+ = - 90 mV)