The solute that contributes the most to the total serum osmolality is:

• A. Glucose
• B. Sodium
• C. Chloride
• D. Urea

Answer: (B)

Total serum osmolality reflects the number of osmotically active particles in plasma. The major contributor is sodium, because extracellular Na+ is the most abundant osmotically active ion and its accompanying anions are counted together, typically represented as 2 × [Na+]. In a normal situation with around 140 mEq/L sodium, this single component accounts for roughly 280 mOsm/kg, which dominates the total. Glucose and urea do contribute, but only to a much smaller extent (glucose about 5 mOsm/kg; urea around 10 mOsm/kg under usual conditions). Chloride isn’t counted separately because its osmotic effect is already included with sodium in the sodium term. So sodium is the solute that contributes the most to total serum osmolality.