There are hazards which aren't in the script at interviews. We've been talking a bit about not digressing, and going off topic yourself.

There's a few things you can do about that:

Isolate the distraction immediately. Answer it, briefly, and kill it off as a subject, then go straight back to what you wanted to say.

Fit it in to your original intended answer, if you can. Looks smooth, and you may actually improve your answer in the process.

If you can't do either of those, ask if you can deal with it separately, so you don't lose track of your answer. Say something like, Look, I'm sorry about this, but I'm trying to make sure I don't louse up my answer to the original question, so can we treat this as a separate question? You're within your rights to do that, and it will give you some breathing space to think about your answer to the new issue.

Handling the unexpected is part of any job, and it's not at all uncommon in job interviews, where someone picks up on some point in your answer.

You'll find that your normal on the job reactions will prevent any trauma. Provided you treat this as yet another unwanted bit of work someone's dumped on you, your reaction to added levels of difficulty will be pretty stable.

That's another good reason for actually saying you want to stay organized. It's a competent response to an unexpected question. Speaking without thinking is almost guaranteed to get you picking up threads again.