Which statement correctly distinguishes crystalloids from colloids?

• A. Crystalloids are proteins; Colloids are salts.
• B. Crystalloids are dissolved salts; Colloids are proteins.
• C. Crystalloids are lipids; Colloids are carbohydrates.
• D. Crystalloids are acids; Colloids are bases.

Answer: (B)

Crystalloids and colloids are distinguished by the size of their solutes and how they distribute in the body. Crystalloids contain small molecules that freely move across capillary walls, so their fluid content shifts quickly between the intravascular space and the interstitial space. Colloids contain larger molecules, such as proteins, that largely stay inside blood vessels and help pull fluid into the vasculature through oncotic effects.

Saying crystalloids are dissolved salts and colloids are proteins reflects this basic distinction: crystalloids are solutions of small solutes (often electrolytes) in water, while colloids contain larger proteins that remain intravascular longer. The other descriptions—crystalloids being lipids or acids, or colloids being carbohydrates or bases—don’t capture that size-based behavior and how it affects fluid distribution.