I don't completely understand what you are trying to do, but I can answer your title question--How do I reference a sprite.
when you click the sprite put the uid in a variable (call it last_pick). You don't change that value until some other sprite from the group is picked at which point last_pick is the uid of that newly picked sprite. This distinquishes one of five otherwise identical sprites as the sprite last picked.
How you store last_pick is a separate question, you could use a static variable, a global variable, a dictionary entry etc...
yours
winkr7