If you calculate the plasma osmolality of a child with plasma Na+ 125 mEq//, glucose 108 mg/ dl, and BUN (blood urea nitrogen) 140 mg/ dl, the most appropriate answer would be:
**Core Concept**
The plasma osmolality is a measure of the concentration of osmotically active particles in the plasma, which is crucial for maintaining proper fluid balance and preventing cellular dehydration or swelling. The calculation of plasma osmolality involves the sum of the concentrations of sodium (Na+), glucose, and blood urea nitrogen (BUN), multiplied by their respective osmolal coefficients (2 for sodium, 1 for glucose, and 2.8 for BUN).
**Why the Correct Answer is Right**
To calculate plasma osmolality, we use the formula: (Na+ concentration in mEq/L) x 2 + (glucose concentration in mg/dL) / 18 + (BUN concentration in mg/dL) x 2.8. Given the values of Na+ = 125 mEq/L, glucose = 108 mg/dL, and BUN = 140 mg/dL, we can plug these values into the formula to get: (125 x 2) + (108 / 18) + (140 x 2.8) = 250 + 6 + 392 = 648 mOsm/kg. This value is within the normal range of plasma osmolality (275-295 mOsm/kg).
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a valid calculation for plasma osmolality.
**Option B:** This option is incorrect because it does not take into account the correct osmolal coefficients for glucose and BUN.
**Option C:** This option is incorrect because it does not provide a valid calculation for plasma osmolality.
**Clinical Pearl / High-Yield Fact**
When calculating plasma osmolality, it's essential to use the correct osmolal coefficients for each substance, as using incorrect coefficients can lead to significant errors in the calculation.
**Correct Answer:**. 648 mOsm/kg