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 potential generated due to freely diffusible ion across a membrane is measured by : 
Select and explain. 
A. Nernst equation, B. Gibbs donnan equillibrium, C. Goldman-katz equation, D. Henderson Hasselbach equation
A: Nernst equation
Answer: A: Nernst equation 
Explanation: Nernst equation: A freely diffusible ion, having different concentrations on either side of a membrane, would diffuse through the membrane and reach equilibrium. Charge created by diffusion on the membrane, when ion reaches equilibrium, can be calculated by Nernst equation. EMF (mV) = +/- 61 X log (C1/C2) C1 and C2: are the concentrations of the ion on either side of the membrane. Goldman-Katz equation is an expanded Nernst equation. It calculates the potential created by more than one ion, when these ions move from their higher to lower concentrations. Henderson Hasselbach equation: used to estimate the pH of a buffer solution Gibbs donnan equillibrium: In the presence of a non- diffusable ion, the diffusable ion distributes themselves so that at equilibrium their concentrations ratios are equal