## **Core Concept**
Plasma osmolality is a measure of the concentration of osmotically active particles in blood plasma. It is primarily determined by the concentration of solutes that cannot freely move across cell membranes. The main contributors to plasma osmolality are sodium and its accompanying anions (mainly chloride and bicarbonate), glucose, and urea.

## **Why the Correct Answer is Right**
Sodium and its accompanying anions (chloride and bicarbonate) are the major contributors to plasma osmolality because they are the most abundant osmotically active particles in the extracellular fluid, including plasma. Sodium is actively regulated by the body and has a very narrow normal range. Due to its active regulation and the fact that it does not freely cross cell membranes, sodium has a significant effect on plasma osmolality.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While glucose can affect plasma osmolality, its concentration is much lower than that of sodium and its effect is usually minimal unless there is significant hyperglycemia, as seen in uncontrolled diabetes mellitus.
- **Option B:** Urea can contribute to osmolality but is considered an ineffective osmole because it can diffuse across cell membranes, thus having a less significant effect on the osmotic gradient across cell membranes compared to sodium.
- **Option C:** This option is not provided, but based on the context, we are focusing on why sodium (often paired with option D in a different format) is the correct answer.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that the normal range for plasma osmolality is approximately 275-295 mOsm/kg. Significant deviations can lead to serious clinical conditions, such as hyponatremia or hypernatremia, which can cause neurological symptoms ranging from confusion to seizures and coma.

## **Correct Answer:** D. Sodium.