Hyponatremia has no effect on the RMP (resting membrane potential) because
## **Core Concept**
The resting membrane potential (RMP) is primarily determined by the distribution and permeability of ions (mainly **sodium (Na+)** and **potassium (K+)**) across the cell membrane, as described by the Nernst equation and the Goldman-Hodgkin-Katz equation. The RMP is generated largely by the **potassium equilibrium potential** due to the high permeability of the membrane to K+ at rest.
## **Why the Correct Answer is Right**
The correct answer, **D.**, implies that the reason hyponatremia has no effect on the RMP is that **sodium ions are not as influential on the resting membrane potential as potassium ions**. This is because, at rest, the cell membrane is less permeable to Na+ compared to K+. The sodium-potassium pump and the cell membrane's selective permeability maintain the RMP, with K+ playing a more significant role. Therefore, changes in external sodium concentration (as in hyponatremia) do not significantly affect the RMP.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not provide a valid reason related to the physiology of RMP generation.
- **Option B:** This option is incorrect because it also does not provide a valid physiological reason for why hyponatremia does not affect RMP.
- **Option C:** This option is incorrect for similar reasons; it lacks a correct physiological basis for the relationship between hyponatremia and RMP.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **the resting membrane potential is largely determined by potassium**, not sodium. This is why changes in external potassium levels have a more significant impact on the RMP than changes in external sodium levels. For example, hyperkalemia can depolarize the membrane, making it easier for neurons to fire.
## **Correct Answer:** **D.**