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
In muscle cells, resting membrane potential is equal to the isoelectric potential of 
Select and explain. 
A. Na+, B. Cl-, C. K+, D. Mg++
C: K+
Answer: C: K+ 
Explanation: The isoelectric potential of Cl- is -70mV and of K+ is -90mV.
According to ‘Ganong’ and ‘Principles of Medical Physiology’, the actual measured RMP of a nerve fibre is -70mV rather than the calculated value of -90mV. 
So in nerve fibres, the RMP is equal to or closer to the isoelectric potential of Cl-.
Whereas in muscle cells, the RMP is -90mV, which is equal to the isoelectric potential of K+.