## **Core Concept**
The resting membrane potential (RMP) of a neuron is primarily determined by the differential distribution of ions (mainly sodium [Na+] and potassium [K+]) across the cell membrane and the selective permeability of the membrane to these ions. This concept is fundamental to understanding neuronal physiology.

## **Why the Correct Answer is Right**
The RMP of a typical neuron is approximately -70 millivolts (mV), which is closest to option **C. -70 mV**. This potential is generated largely by the movement of potassium ions out of the cell, which creates a negative charge inside the cell relative to the outside. The **sodium-potassium pump** also plays a critical role by actively transporting 3 sodium ions out and 2 potassium ions into the cell, using ATP for energy. However, the immediate cause of the RMP is the high permeability of the membrane to potassium ions (due to **leakage channels**) and the concentration gradient for potassium.

## **Why Each Wrong Option is Incorrect**
- **Option A:** -50 mV is more positive than the typical RMP of a neuron. While certain neurons or specific conditions might alter the RMP, -50 mV does not accurately represent the standard RMP.
- **Option B:** -90 mV is more negative than the typical RMP. Although some neurons might have RMPs in this range, -70 mV is a more universally accepted value for the RMP of neurons.
- **Option D:** -100 mV is significantly more negative than the RMP of most neurons. This value might be seen in certain types of cells or under specific experimental conditions but not typically for neurons.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the RMP of neurons can be influenced by various factors including the **concentration of ions** (especially K+ and Na+), **temperature**, and the presence of certain **neurotransmitters** that can modulate ion channels. Understanding how these factors affect the RMP is crucial for comprehending neuronal excitability and synaptic transmission.

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