This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cell Membrane and transpo protiens, Subject name: Physiology
A muscle cell has an intracellular of 14 mM and an extracellular of 140 mM. Assuming that 2.3 /F = 60 mV, what would the membrane potential be if the muscle cell membrane were permeable only to Na+? 
Select and explain. 
A. +80 mV, B. -60 mV, C. -80 mV, D. +60 mV
D: +60 mV
Answer: D: +60 mV 
Explanation: Nernst equation Used to calculate the equilibrium potential for a single ion. In applying the Nernst equation, we assume that the membrane is freely permeable to that ion alone. ENa+ = 2.3 /zF log Ce/Ci ENa+ = 60 mV log 140/14 = 60 mV log 10 ENa+= 60 mV. Notice that the signs were ignored and that the higher concentration was simply placed in the numerator to simplify the log calculation. To determine whether ENa+ is +60 mV or -60 mV,- Use the intuitive approach= Na+ will diffuse from extracellular to intracellular fluid down its concentration gradient, making the cell interior positive.