In a hospital error, a 60-year-old woman is infused with large volumes of a solution that causes lysis of her red blood cells (RBCs). The solution was most likely

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[III B 2 d]. Lysis of the patient's red blood cells (RBCs) was caused by entry of water and swelling of the cells to the point of rupture. Water would flow into the RBCs if the extracellular fluid became hypotonic (had a lower osmotic pressure) relative to the intracellular fluid. By definition, isotonic solutions do not cause water to flow into or out of cells because the osmotic pressure is the same on both sides of the cell membrane. Hypertonic solutions would cause shrinkage of the RBCs. $150 \mathrm{mM} \mathrm{NaCl}$ and $300 \mathrm{mM}$ mannitol are isotonic. $350 \mathrm{mM}$ mannitol and $150 \mathrm{mM} \mathrm{CaCl}_{2}$ are hypertonic. Because the reflection coefficient of urea is $<1.0,300 \mathrm{mM}$ urea is hypotonic.

In a Hospital Error, a 60-Year-Old Woman Is Infused with Large