## **Core Concept**
The Nernst equation is used to calculate the equilibrium potential (also referred to as the Nernst potential) for an ion. This equation takes into account the concentration of the ion inside and outside the cell, as well as the charge of the ion. For chloride ions (Cl–), the equilibrium potential can be calculated using the Nernst equation at human body temperature (approximately 37°C).

## **Why the Correct Answer is Right**
The Nernst equation for an ion at human body temperature (37°C or 310 K) can be simplified to:
[ E_{ion} = frac{61.54}{z} log left( frac{[ion]_{outside}}{[ion]_{inside}} right) ]
For chloride ions (Cl–), (z = -1). Typical concentrations for chloride ions are approximately 110 mM outside the cell and 20 mM inside the cell. Substituting these values into the equation gives:
[ E_{Cl} = frac{61.54}{-1} log left( frac{110}{20} right) ]
[ E_{Cl} = -61.54 log (5.5) ]
[ E_{Cl} = -61.54 times 0.740 ]
[ E_{Cl} approx -61.54 times 0.74 ]
[ E_{Cl} approx -45.5 , mV ]
This matches option , which is approximately -90/2 or -45 mV when rounding to a simple calculation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option does not match the calculated Nernst potential for chloride.
- **Option B:** This option suggests a positive value, which is incorrect for chloride given its typical concentration gradient across the cell membrane.
- **Option D:** This option does not align with the calculated value for the chloride equilibrium potential.

## **Clinical Pearl / High-Yield Fact**
The chloride equilibrium potential is important in understanding the physiology of neurons and muscle cells. Chloride channels play a critical role in stabilizing the membrane potential and in the regulation of excitability. The calculated equilibrium potential for chloride helps in understanding how chloride conductance can lead to hyperpolarization or stabilization of the membrane potential.

## **Correct Answer:** .