I have had it a couple of times with chicks I got at the feed store. I completely agree with azygous for the treatment. Keep an eye one it and look for it with any future chicks.
Is there poop dried on. If so remove it with a warm wet cotton ball. You might put some prepH and try to massage it back into place. Use a thin rubber glove on the hand doing the massaging. The chick will die if the blockage is not cleaned. It looks to me like it is dried poop on the vent...