## **Core Concept**
The resting membrane potential (RMP) is the difference in electrical charge between the inside and outside of a neuron when it is not being stimulated. This potential is generated by the distribution of ions (mainly sodium [Na+] and potassium [K+]) across the cell membrane and the selective permeability of the membrane to these ions. The RMP is crucial for the proper functioning of neurons, including the generation and propagation of action potentials.

## **Why the Correct Answer is Right**
The resting membrane potential of neurons is primarily established by the **sodium-potassium pump** and the **selective permeability** of the neuronal membrane. The sodium-potassium pump actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell, using ATP for energy. The neuronal membrane is more permeable to potassium ions than to sodium ions at rest, allowing potassium ions to leave the cell more easily. This movement of positively charged ions out of the cell results in a net negative charge inside the cell compared to the outside. The typical value for the resting membrane potential of neurons is about **-70 mV**, which is close to the equilibrium potential for potassium.

## **Why Each Wrong Option is Incorrect**
- **Option A:** -90 mV is more negative than the typical resting membrane potential of neurons. While some neurons may have RMPs around this value, it is not the commonly cited average.
- **Option B:** -50 mV is less negative than the typical RMP, which might be seen in certain conditions but is not standard for a resting neuron.
- **Option D:** -100 mV is more negative than the usual range for the RMP of neurons, and such a value is not typically observed in neurons at rest.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the resting membrane potential is mainly determined by the **potassium equilibrium potential** due to the high permeability of the resting membrane to potassium. Alterations in the RMP can affect the excitability of neurons. For example, depolarization (becoming less negative) can make it easier for a neuron to fire an action potential, while hyperpolarization (becoming more negative) can make it harder.

## **Correct Answer:** . -70 mV