## **Core Concept**
The Nernst potential, also known as the equilibrium potential, is the membrane potential at which the electrical and chemical forces acting on an ion are balanced. It is calculated using the Nernst equation, which takes into account the concentration gradient of the ion across the cell membrane. For potassium (K+), the Nernst potential is typically around -90 mV.

## **Why the Correct Answer is Right**
The Nernst equation for an ion at human body temperature (37°C) can be simplified to: (E_{ion} = frac{61.54}{z} logleft(frac{[ion]_{outside}}{[ion]_{inside}}right)) mV, where (z) is the charge of the ion, and ([ion]_{outside}) and ([ion]_{inside}) are the concentrations of the ion outside and inside the cell, respectively. For K+, (z = +1), ([K^+]_{outside} approx 5) mM, and ([K^+]_{inside} approx 150) mM. Substituting these values into the equation yields: (E_{K+} = 61.54 logleft(frac{5}{150}right) = 61.54 logleft(frac{1}{30}right) = 61.54 times -1.477 = -90) mV. This matches option.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This value does not match the calculated Nernst potential for K+.
- **Option B:** This is closer but still not the accurate Nernst potential for K+; it might be a rounded or incorrect value.
- **Option D:** This value seems too positive for the Nernst potential of K+.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the resting membrane potential of most neurons is close to the Nernst potential for K+, around -90 mV, because the cell membrane is more permeable to K+ at rest due to the presence of leak potassium channels. This helps in understanding how the resting membrane potential is generated and maintained.

## **Correct Answer:** .