## **Core Concept**
The question tests understanding of the Nernst equation and the effect of ion concentration changes on the resting membrane potential, specifically focusing on potassium (K+) permeability. The resting membrane potential is largely determined by the movement of K+ ions out of the cell.

## **Why the Correct Answer is Right**
The Nernst equation for an ion at human body temperature (37°C) is given by (E_{ion} = frac{61.54}{z} logleft(frac{[ion]_{outside}}{[ion]_{inside}}right)), where (z) is the charge of the ion. For K+, (z = +1). Initially, with ([K^+]_{outside} = 5) mM and ([K^+]_{inside} = 140) mM, the Nernst potential for K+ ((E_K)) is (E_K = 61.54 logleft(frac{5}{140}right) = 61.54 log(0.0357) = 61.54 times -1.447 = -86.3) mV. If ([K^+]_{outside}) is reduced to 2.5 mM, then (E_K = 61.54 logleft(frac{2.5}{140}right) = 61.54 log(0.0179) = 61.54 times -1.747 = -107.4) mV. Reducing the extracellular K+ concentration makes the Nernst potential for K+ more negative.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests that reducing extracellular K+ concentration would depolarize the membrane. However, decreasing ([K^+]_{outside}) makes (E_K) more negative, which would actually hyperpolarize the membrane if the cell were only permeable to K+, not depolarize it.
- **Option B:** This option suggests no change, which contradicts the calculation that shows a change in (E_K) when ([K^+]_{outside}) changes.
- **Option C:** This option suggests a less negative membrane potential, which is equivalent to saying depolarization occurs. However, as explained, reducing ([K^+]_{outside}) makes the membrane potential more negative (hyperpolarization), not less.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the resting membrane potential is primarily determined by K+ permeability. According to the Nernst equation, decreasing the extracellular K+ concentration will make the inside of the cell more negative relative to the outside, causing hyperpolarization. This concept is crucial for understanding how changes in ion concentrations affect neuronal excitability.

## **Correct Answer:** .