Which solution should NOT be given through an existing IV line when administering blood products?

When administering blood products through an existing IV line, using dextrose solutions is discouraged because they can cause hemolysis (breakdown of red blood cells) when mixed with blood components. Dextrose solutions, particularly in higher concentrations, create an environment that is less compatible with blood products, potentially compromising their effectiveness and safety.

In contrast, normal saline is often used to flush IV lines before and after starting blood transfusions, as it is isotonic and does not interact adversely with blood. The same goes for Lactated Ringer’s solution, which is also compatible with blood products but isn't typically the first choice for blood administration. Using 5% dextrose in water poses similar risks as the dextrose solutions since it contains sugar, which could lead to hemolytic reactions if mixed with blood.

Therefore, among the choices, dextrose solutions are the least appropriate option for administration through an IV line already in use for blood products due to the risk of hemolysis and adverse reactions.