Yeah, I second that. "I had a medical issue while I was in school that has since been resolved" is a good, non-specific way of communication that yes, there is a good reason for your messy transcript, and no, it's not an issue any more.
They *shouldn't* push you any more after that, but just in case they do: spend some time thinking about whether you'd rather tell them "I used to have trouble with anxiety, but it's been under control for several years" or if you'd rather say "I'd rather not discuss specifics, thanks."