Which of the following examples is NOT considered an isotonic solution?

D5W, or dextrose in water, is not considered an isotonic solution when looking at physiological effects. While D5W starts as an isotonic solution at a concentration of 5% dextrose in water, once it is administered and metabolized, the dextrose is quickly utilized by the body's cells, leading to a net hypotonic effect. This means that the solution, after metabolism, can result in a lower osmolarity compared to plasma, potentially causing fluid shifts.

In contrast, isotonic solutions like sodium chloride and lactated Ringer's solution maintain the same osmotic pressure as bodily fluids, ensuring that there is no net movement of water in or out of cells. Albumin, while not technically a saline solution, functions effectively as a volume expander and operates similarly in that it helps to maintain osmotic balance within the vascular space. Thus, it does not become hypotonic once introduced.

D5W's unique metabolic pathways differentiate it as a solution not consistently maintaining isotonic characteristics in practice, providing clarity on its classification compared to the other solutions listed.