The little chick looks very young so may be having a case of pasty butt. I would run a sink of water just deep enough that I could get the little ones bottom wet and gently clean the bottom with a paper towel. Then keep a close watch on it so you know for sure if it is able to poo or not. The protrusion may be from it straining to poo or from the irritation of the poo being stuck.
Keep a watch on the vent to see if it recovers or not. I personally do not think this chick is a lost cause yet.
Some chicks have a rough trip and have this happen as well as some chicks simply cannot clear the feathers when they poo.
I would clean it as needed (any time you see it with poo stuck on) and make sure it is warm enough as well as that it is eating and drinking.
I have had pasty butt happen to a polish that had a horrid trip and had to keep cleaning her for almost 3 weeks. Poor little dear was stuck in transit an extra 2 days.
Edited by 21hens-incharge - 2/12/16 at 3:30pm