How do I tell my roommate that her room smells?

I live in an apartment with two other girls. One of the girls (the one who has lived in the apartment the longest) does not clean. The other roommate and I have accepted this fact and don’t mind as long as she doesn’t make it too much dirtier.

The problem is that she doesn’t clean her room, and it stinks to high heaven. I went in there today to feed her cat and I had to hold my shirt over my nose, it was so bad. I don’t even know what the origin of the smell is (dirty laundry? cat litter? rotting food? all of the above?).

So, how do I go about showing/telling/hinting to her that her room smells really really bad?