How to remove lower spray arm

Our dishwasher has suddenly stopped getting our dishes as clean and is not fully draining at the end of a wash. I am sure that it is just plugged, but I cannot for the life of me figure out how to get the sprayer arm removed to clean it out. It looks like a screw in piece that goes through the arm into the sprayer hub, and while I can get the piece at the top to turn (with much difficulty and resistance), I cannot get the arm free. I am not sure, however, if it is actually supposed to screw out or not, and I do not want to break the hub in the process. Any help here would be greatly appreciated! Thanks.