I wouldn't necessarily equate a skin condition with a disability, but I get what you mean. In the moment, if my 3 year old comments on a difference, I usually say something like, "sometimes people are different from each other, and that's okay. How beautiful skin can be! I wish I had more freckles!" etc.
Dd is 4 and usually I just say a quick "because people are different" until we can talk about it more. It's hard because sometimes the difference is an illness or wound and sometimes it's something more permanent. I have also said in a pinch "oh I'm not sure" while we are in the room and then we might talk more in the car.
If it's something similar to what she's had, I will say "oh you've had boo boos like that." And then we'll start talking about her injuries and suddenly we are down that road and the spotlight is off the stranger.