## Core Concept
Plasma osmolality is a measure of the concentration of osmotically active particles in blood plasma. It is calculated using the concentrations of sodium, glucose, and blood urea nitrogen (BUN), as these are the major contributors to osmolality. The formula to calculate plasma osmolality is: Osmolality (mOsm/kg) = (2 x [Na+]) + (glucose / 18) + (BUN / 2.8), where sodium concentration is in mmol/L, glucose in mg/dL is divided by 18 to convert to mmol/L, and BUN in mg/dL is divided by 2.8 to convert to mmol/L.

## Why the Correct Answer is Right
To calculate plasma osmolality using the given values: serum Na+ is 130 mmol/L (assuming it's given in mmol/L, as it's common), glucose is 180 mg/dL, and BUN is 56 mg/dL. Since K+, Cl- are not directly included in the basic formula, we focus on Na+, glucose, and BUN. The calculation is: Osmolality = (2 x 130) + (180 / 18) + (56 / 2.8) = 260 + 10 + 20 = 290 mOsm/kg.

## Why Each Wrong Option is Incorrect
- **Option A:** Without calculating, we cannot assume it's incorrect, but let's evaluate other options.
- **Option B:** Similarly, needs evaluation.
- **Option C:** This seems to align with our manual calculation (290 mOsm/kg), suggesting other options might be incorrect based on miscalculation or incorrect assumptions.
- **Option D:** Will be assessed based on calculation accuracy.

Given our calculation (290 mOsm/kg), any option significantly different from this calculation would be incorrect based on the arithmetic.

## Clinical Pearl / High-Yield Fact
A key point to remember is that normal plasma osmolality ranges approximately from 275 to 295 mOsm/kg. Values outside this range can indicate various conditions such as dehydration (hyperosmolar) or water intoxication (hyposmolar).

## Correct Answer: C. 290.