## Core Concept
Normal saline, also known as isotonic saline, is a 0.9% solution of sodium chloride (NaCl) in water. It is isotonic to most bodily fluids, meaning it has the same concentration of solutes as human blood. The concentration of sodium (Na+) in normal saline is crucial for understanding its use in clinical settings.

## Why the Correct Answer is Right
Normal saline is made by dissolving 0.9 grams of NaCl in 100 milliliters of water, which results in a solution that is 154 mEq/L of Na+ and 154 mEq/L of Cl-. Since NaCl dissociates into one sodium ion (Na+) and one chloride ion (Cl-), the concentration of sodium ions in normal saline is 154 mEq/L. This matches option **C. 154**, making it the correct answer.

## Why Each Wrong Option is Incorrect
- **Option A: 140** - This concentration is close to the normal sodium level in human blood (135-145 mEq/L) but does not accurately reflect the concentration of sodium in normal saline.
- **Option B: 145** - This is slightly below the actual sodium concentration in normal saline.
- **Option D: 120** - This is significantly lower than the actual concentration of sodium in normal saline and does not match any standard solution used in clinical practice.

## Clinical Pearl / High-Yield Fact
A key point to remember is that normal saline (0.9% NaCl solution) has a sodium concentration of 154 mEq/L. This is important for calculating fluid balances, administering medications, and understanding the osmolarity of intravenous solutions.

## Correct Answer: C. 154