## **Core Concept**
Isotonic saline, also known as 0.9% saline solution, is a solution that has the same osmolarity as blood. It is commonly used in medical settings for fluid resuscitation and as a vehicle for medication administration. The osmolarity of isotonic saline is approximately 308 mOsm/L.

## **Why the Correct Answer is Right**
One liter of isotonic saline contains 9 grams of sodium chloride (NaCl), which is equivalent to 0.9% solution. The molecular weight of NaCl is 58.44 g/mol. When dissolved in water, NaCl dissociates into two ions: sodium (Na+) and chloride (Cl-). The molar concentration of a 0.9% NaCl solution can be calculated as follows: 9 g/L / 58.44 g/mol = 0.154 mol/L. Since NaCl dissociates into two ions, the concentration of sodium ions (Na+) is 0.154 mol/L. To convert this into milliequivalents per liter (mEq/L), we use the fact that the valence of sodium is 1. Therefore, 0.154 mol/L * 1 * 1000 = 154 mEq/L.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a sodium content that is lower than what is calculated for isotonic saline.
- **Option B:** This option is incorrect because it underestimates the sodium content of isotonic saline.
- **Option D:** This option overestimates the sodium content of isotonic saline.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that isotonic saline has a sodium concentration close to that of human blood, which makes it suitable for intravenous administration without causing significant shifts in electrolyte balance. The typical sodium concentration in isotonic saline is approximately 154 mEq/L.

## **Correct Answer:** . 154 mEq/L