**Question:** If you calculate the plasma osmolality of a child with plasma Na+ 125 mEq/l, glucose 108 mg/ dl, and BUN (blood urea nitrogen) 140 mg/ dl, the most appropriate answer would be:

A.
B.
C.
D.

**Correct Answer:**

**Core Concept:** Osmolality is a measure of the concentration of solute particles, mainly osmotically active substances like sodium, glucose, and urea, in a given volume of plasma or serum. In this context, plasma osmolality is calculated by dividing the sum of the concentrations of these solutes by the osmotic coefficient (33.8 for plasma).

**Why the Correct Answer is Right:**

Based on the provided values, we can calculate the plasma osmolality as follows:

1. Plasma sodium (Na+) concentration is 125 mEq/l.
2. Glucose concentration is 108 mg/dl.
3. Blood urea nitrogen (BUN) concentration is 140 mg/dl.

The osmotically active substances are:

- Sodium (Na+)
- Glucose
- Urea (urea)

Using the concentrations provided, we can calculate the plasma osmolality:

Plasma osmolality = (Na+ concentration + Glucose concentration + Urea concentration) / Osmotic coefficient (33.8)

Plasma osmolality = (125 + 108 + 140) / 33.8
Plasma osmolality = 373 / 33.8
Plasma osmolality = 11.09 Osmol/kg

**Why Each Wrong Option is Incorrect:**

A. Plasma osmolality is not affected by BUN levels since urea is not a significant contributor to osmolality compared to glucose and sodium.
B. The calculated plasma osmolality does not change based on the presence or absence of option C (glucose), as it is considered a major contributor to plasma osmolality.
D. Osmolality is not influenced by the absolute values of Na+ and glucose, as the osmotic coefficient (33.8) accounts for the total osmotically active substances.

**Clinical Pearls:**

1. Osmolality is a measure of the solute concentration in plasma and is influenced by the presence and concentration of sodium, glucose, and urea.
2. The osmotic coefficient (33.8) adjusts for the total osmotically active substances, ensuring that osmolality reflects the combined effect of these solutes.
3. The significance of BUN (Blood Urea Nitrogen) in determining plasma osmolality is minimal due to its relatively low contribution to osmolality compared to glucose and sodium.

**Why the Correct Answer is Right:**

The correct answer is **Option A (Plasma osm