The guideline is that you may not have:
1.1.4 Overtly sexual or pornographic material, defined by Webster's Dictionary as "explicit descriptions or displays of sexual organs or activities intended to stimulate erotic rather than aesthetic or emotional feelings."
I suspect that is not an issue provided the user is "authorized" and not a group of 12 year olds. Blurring may actually be more titillating then the real image (e.g. hemorroid images). I'd not show certain images if the user is not "authorized" and point out to App Review how your authorization procedure limits the user to medically interested. However, all apps must do something so you can't lock-out an unauthorized user.