Which of the following is NOT among the three most osmotically active particles in serum?

• A. Sodium
• B. Glucose
• C. BUN
• D. Potassium

Answer: (D)

The major determinants of serum osmolality are the solutes present in high extracellular concentration: the sodium ion, glucose, and urea (BUN). Sodium sets the bulk of the osmotic pressure outside cells, while glucose and urea also contribute notably to the total osmolality. Potassium, though essential, is normally present at far lower extracellular levels and is largely intracellular, so its contribution to serum osmolality is comparatively small. That’s why potassium is not considered one of the three most osmotically active particles. In practice, osmolality is estimated mainly from sodium, glucose, and BUN, with potassium not included in the standard calculation.