**Question:** Ratio of concentration of the sodium ions inside the cells to that of outside the cells

**Core Concept:** The cell membrane potential, also known as resting membrane potential, is primarily determined by the concentration gradient of sodium (Na+) and potassium (K+) ions across the cell membrane. Maintaining this gradient is essential for proper cell function and communication between cells.

**Why the Correct Answer is Right:** The correct answer, **Option C: 3:1**, represents the ratio of sodium ions (Na+) inside the cells to those outside the cells. This ratio is crucial for maintaining the resting membrane potential, which is essential for the proper functioning of neurons and muscle cells.

Inside the cell, sodium ions are concentrated due to the activity of the sodium-potassium pump (Na+/K+ ATPase), which actively transports three sodium ions out of the cell for every two potassium ions that enter the cell. This maintains the high concentration of sodium ions inside the cell and low concentration outside the cell, contributing to the resting membrane potential.

**Why Each Wrong Option is Incorrect:**

**Option A (2:1):** This ratio is incorrect because it does not accurately represent the concentration gradient of sodium ions. A 2:1 ratio would suggest that there are twice as many sodium ions inside the cell as outside, which is not the case.

**Option B (1:1):** This ratio is incorrect as it does not reflect the difference in sodium concentrations between the cell and extracellular fluid. A 1:1 ratio would imply that there is equal concentration of sodium ions on both sides of the cell membrane, which is not true.

**Option D (5:1):** This ratio is incorrect because it significantly overestimates the concentration of sodium ions inside the cell. A 5:1 ratio would suggest that there are five times as many sodium ions inside the cell compared to outside, which is not the case.

**Clinical Pearl:** The correct ratio of sodium ions concentration is essential for maintaining the resting membrane potential and proper cellular function. Dysregulation of this ratio can lead to pathophysiological conditions such as epilepsy, cardiac arrhythmias, and neurotransmitter excitability. For example, in epilepsy, the increased sodium ion influx can lead to uncontrolled neuronal activity, while in cardiac arrhythmias, abnormal sodium ion concentration can cause abnormal heart rhythms.

**Correct Answer Explanation:** The correct answer, **Option C (3:1)**, reflects the activity of the sodium-potassium pump (Na+-K+ ATPase) responsible for actively transporting sodium ions out of the cell and potassium ions into the cell, maintaining the concentration gradient necessary for proper cellular function and membrane potential.