He went to go get lightbulbs. [Does that explain why Kuzuryuu has one? Not really, but there's no way it's unrelated.] The hallway he took us down to get to the containment facility was too dark to see anything. But after he left, the stairs gave out beneath us and we slid the rest of the way down.

[Okay, so he might actually have some very light bruises from that but since he hasn't had a chance to get undressed yet, not even Kuzuryuu himself knows for sure.]