Which of the following is an isotonic crystalloid?

• A. Lactated Ringer's Solution
• B. Hypertonic saline
• C. Dextrose 5% in water
• D. Hypotonic saline

Answer: (A)

Isotonic crystalloids have osmolality close to that of plasma, so they increase circulating blood volume without causing large shifts of fluid into or out of cells. Lactated Ringer’s Solution fits this profile: its electrolyte content and osmolarity are similar to plasma, so it stays mainly in the intravascular space and effectively expands volume during dehydration or bleeding.

Hypertonic saline, by contrast, has a higher osmolality and pulls water from cells into the bloodstream, which is useful in specific situations but not isotonic with plasma. Dextrose 5% in water behaves like a crystalloid at first, but once the glucose is metabolized, it leaves free water and becomes effectively hypotonic, so it’s not an isotonic crystalloid. Hypotonic saline has lower osmolality than plasma and drives water into cells, not ideal for rapid intravascular volume expansion.

So the isotonic crystalloid among these options is Lactated Ringer’s Solution.