I encountered this Tsumego on the Tsumego Pro app. I’ve already solved it, in three different variations; the thing is, I think I can Tenuki in one of the variations before the sequence is finished, but the app says I’m wrong.
For those who aren’t familiar with the app, when it says you’re wrong, it stops responding to your moves; so I don’t know which moves could white do to kill black in the Tenuki variations. But as you can see in the demo, I tried a few of them, and they all ended in white getting killed. Also, I don’t think Ko is involved.