When a client has gluteal edema, why should the nurse avoid using the gluteus maximus muscle for administration of intramuscular medications?

With gluteal edema, it's never advisable to give an IM injection. It'll be better to administer the injection in the lateral thigh. The reason why it's not a good idea to administer IM into ANY edema is because the probability of the injection actually going intramuscularly is very slim. You'd essentially only be giving medication into the edematous fluid; which obviously is going to render the injection ineffective.