Which of the following is an example of an isotonic crystalloid?

• A. Lactated Ringer's Solution
• B. Dextrose 5% in water
• C. 0.25% NaCl
• D. 0.9% NaCl

Answer: (A)

Isotonic crystalloids have an osmolality close to that of plasma, so they expand the extracellular fluid without shifting water into or out of cells. Lactated Ringer’s fits this idea well because its osmolarity is near plasma and it supplies a balanced mix of electrolytes (sodium, potassium, calcium) with lactate that acts as a buffer. This makes it a reliable fluid for volume resuscitation while keeping cells from swelling or shrinking.

Dextrose 5% in water behaves differently: the glucose is rapidly taken up by cells, leaving behind water that can enter cells and dilute the extracellular space, so it doesn’t stay truly isotonic over time. The 0.25% sodium chloride solution is hypotonic, with too little solute to match plasma osmolality. The 0.9% sodium chloride solution is typically considered isotonic as well, but it has a higher chloride content and lacks the balanced electrolyte composition of LR; in many clinical contexts LR is preferred for not only isotonicity but also closer match to plasma electrolyte balance and buffering capacity.