## Core Concept
The plasma osmolality can be calculated using the formula: Osmolality (mOsm/kg) = (2 x Na+) + (glucose / 18) + (BUN / 2.8), where Na+ is in mEq/L, glucose in mg/dL, and BUN in mg/dL. This formula accounts for the major osmotically active particles in plasma: sodium (and its accompanying anions), glucose, and urea.

## Why the Correct Answer is Right
To calculate the plasma osmolality for the given values: Na+ = 125 mEq/L, glucose = 108 mg/dL, and BUN = 140 mg/dL, we substitute these into the formula.
- For sodium: 2 x 125 = 250
- For glucose: 108 / 18 = 6
- For BUN: 140 / 2.8 = 50
Adding these together: 250 + 6 + 50 = 306 mOsm/kg.

## Why Each Wrong Option is Incorrect
- **Option A:** Without calculating, we cannot assume it's correct or incorrect, but let's assume a value less than 306. If the values calculated do not add up to at least 306, this option would be incorrect.
- **Option B:** Similarly, if this option suggests a value not equal to 306 mOsm/kg based on our calculation, it would be incorrect.
- **Option D:** This option would be incorrect if its value does not match 306 mOsm/kg.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the normal plasma osmolality range is approximately 275-295 mOsm/kg. The patient's calculated osmolality of 306 mOsm/kg indicates hyperosmolality, which can occur due to various reasons such as hyperglycemia, elevated BUN levels (as seen in this case), or other solute imbalances.

## Correct Answer: C. 306.