## **Core Concept**
The electrochemical driving force for an ion across a cell membrane is determined by the difference between the membrane potential and the equilibrium potential for that ion. The resting membrane potential of a typical neuron is around -65 millivolts. The equilibrium potentials for various ions depend on their concentration gradients across the membrane.

## **Why the Correct Answer is Right**
The equilibrium potential for an ion can be calculated using the Nernst equation. For a typical neuron:
- The equilibrium potential for potassium (EK) is approximately -90 millivolts, because the concentration gradient for potassium is such that it favors potassium leaving the cell.
- The equilibrium potential for sodium (ENa) is approximately +60 millivolts, as sodium's concentration gradient favors its entry into the cell.
- The equilibrium potential for chloride (ECl) is around -70 millivolts, as chloride's distribution is such that it is slightly more concentrated outside the cell.

Given these values:
- For potassium (K+), the driving force is |(-65) - (-90)| = 25 millivolts.
- For sodium (Na+), the driving force is |(-65) - 60| = 125 millivolts.
- For chloride (Cl-), the driving force is |(-65) - (-70)| = 5 millivolts.

The ion with the lowest electrochemical driving force in this scenario is chloride (Cl-), as it has the smallest difference between the resting membrane potential and its equilibrium potential.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Incorrect because, although potassium has a significant driving force, it is not the lowest; chloride has a lower driving force.
- **Option B:** Incorrect because sodium has a very high driving force due to its large concentration gradient and the resting membrane potential being far from its equilibrium potential.
- **Option D:** Incorrect because, similar to sodium, this option would represent an ion (usually calcium, Ca2+) with a significant driving force, but not the lowest.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the resting membrane potential is closest to the equilibrium potential for potassium, which helps explain why potassium is the ion with the most significant influence on the resting membrane potential. However, in terms of driving force, chloride often has a relatively low driving force near the resting state, which can make it crucial for certain types of synaptic inhibition.

## **Correct Answer:** .