Which solution is isotonic relative to plasma?

• A. 0.23% NaCl
• B. 0.9% NSS
• C. 5% NaCl
• D. 10% H2O

Answer: (B)

Isotonic means having the same osmolality as plasma, so it stays in the extracellular space without shifting water into or out of cells. Normal saline, a 0.9% NaCl solution, matches plasma osmolality and distributes primarily in the extracellular fluid, making it isotonic. The other options are either hypotonic or hypertonic relative to plasma: a solution with lower solute concentration pulls water into cells and can cause swelling; a solution with higher solute concentration pulls water out of cells and can cause shrinkage; pure water has virtually no solutes and would rapidly cause cells to take in water. Therefore, the isotonic choice is normal saline.