How old is this woman.....seven???

Poke her with the tip of a pencil and tell her that's her cootie shot. Now she's safe!!
I'd give her the website address of the NMSS and (if she can read) tell her to look it up and educate herself on it!!
People like her just irritate the daylights out of me (can you tell??

).
You don't owe anyone an explanation, Joelle. I know you're doing it for you son and I totally understand that....but people also have a responsibility to educate themselves on things that are unfamiliar to them.
If she knows you have MS and knows that she is going to meet you then, IMHO, she should at least do some research on the condition so she doesn't come across as a total moron. I would if I was meeting someone with a condition I wasn't familiar with and knew about the meeting ahead of time.