X, Y and Z are three permeable ions. If at resting potential, X and Y are – 50 and – 30 respectively. What is the value of Z?

**Core Concept**
The resting membrane potential is determined by the concentration gradients and permeability of ions across the cell membrane. At resting potential, the net movement of ions is from areas of higher concentration to areas of lower concentration, resulting in a negative charge inside the cell.

**Why the Correct Answer is Right**
According to the Nernst equation, the resting membrane potential (E_m) is given by the equation: E_m = (RT/F) * ln([K^+](outside)/[K^+](inside)) for potassium ions, but since we are not given the values for X, Y and Z, we can use a simpler approach. The resting membrane potential is approximately equal to the Nernst potential for the most permeable ion. Since X and Y are -50 and -30, respectively, we can assume that X is more permeable. Let's assume that Z is the Nernst potential for the most permeable ion, which is X. Since X is -50, we can assume that Z is also -50.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the value of Z is not given and we cannot assume it to be a fixed value.
* **Option B:** This option is incorrect because the value of Y is not equal to the value of Z.
* **Option C:** This option is incorrect because the value of X is not equal to the value of Z.
* **Option D:** This option is incorrect because the value of Z is not given and we cannot assume it to be a fixed value.

**Clinical Pearl / High-Yield Fact**
The Nernst equation can be used to calculate the resting membrane potential for different ions, but in practice, the most permeable ion is usually the one that determines the resting membrane potential.

**Correct Answer:** C. -50