## Core Concept
Isotonic solutions have the same concentration of solutes as blood plasma, which is approximately 0.9% sodium chloride (NaCl) or 154 mEq/L of solutes. These solutions neither cause cells to swell nor shrink when they are in contact with the cells. The key concept here is the comparison of solute concentrations.

## Why the Correct Answer is Right
The correct answer, . or 0.45% NaCl, is not isotonic because it has a lower concentration of solutes compared to blood plasma. It is considered a hypotonic solution because it causes cells to swell when in contact with them. This solution has half the solute concentration of isotonic saline (0.9% NaCl), making it inappropriate for intravenous administration when an isotonic solution is required.

## Why Each Wrong Option is Incorrect
- **Option A:** is isotonic to blood plasma. It has the same solute concentration as plasma and does not cause a net movement of water into or out of cells.
- **Option B:** is isotonic. Although it contains multiple solutes, its overall osmolarity is similar to that of blood plasma, making it isotonic.
- **Option D:** (lactated Ringer's solution) is isotonic and has a composition close to that of blood plasma, with the addition of lactate which helps in buffering.

## Clinical Pearl / High-Yield Fact
A key point to remember is that 0.9% NaCl (normal saline) and lactated Ringer's solution are commonly used isotonic solutions in clinical practice. Solutions that are not isotonic can cause hemolysis (if hypotonic) or shrinkage of cells (if hypertonic). Always verify the tonicity of a solution before administration, especially intravenously.

## Correct Answer: .