## Core Concept
The concentration of sodium in plasma is a critical aspect of electrolyte balance in the body. Sodium is the most abundant cation in the extracellular fluid, and its concentration is tightly regulated. The normal range of sodium in plasma is essential for various physiological functions, including nerve and muscle function.

## Why the Correct Answer is Right
The correct concentration of sodium in plasma is approximately **135-145 mmol/L**. This range is crucial for maintaining proper osmotic balance and facilitating various bodily functions. Sodium plays a vital role in the transmission of nerve impulses and the contraction and relaxation of muscles. The body tightly regulates sodium levels through various mechanisms, including renal excretion and hormonal control.

## Why Each Wrong Option is Incorrect
* **Option A:** 120-130 mmol/L - This range is below the normal concentration of sodium in plasma. Levels below 135 mmol/L are considered hyponatremia, which can lead to serious health issues.
* **Option B:** 150-160 mmol/L - This range is above the normal concentration of sodium in plasma. Levels above 145 mmol/L are considered hypernatremia, which can also lead to serious health issues.
* **Option D:** 100-110 mmol/L - This range is significantly below the normal concentration of sodium in plasma and would be considered a severe case of hyponatremia.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the normal sodium level in plasma is **135-145 mmol/L**. Significant deviations from this range can lead to conditions such as hyponatremia or hypernatremia, both of which require prompt medical attention.

## Correct Answer: B. 135-145 mmol/L.