Dear Lostinthere, you can say something like, "She helps me out with medical problems." Also, unless they are a business proprieter or someone officially given the job to guard a door, in general, your average person can be politely (in so many words) told to mind their own business, by giving a very general answer such as the one that I just presented above.
Should they get nosey and question you further, it is time for you to start asking the questions! Ask if they have some OFFICIAL reason for which they need to know. Or tell them, "Listen, I already told you and I am running late, please excuse me". Then leave! Really, curiosity is not an excuse.
Now they may want to know, in order to see if they qualify for a dog. Or maybe if their child quailifies. Then it is time, again, to ask your questions. "What condition(s) does the child have that would necessetate a Helper dog?" is something you might want to ask them. And if that is something you know little about, you can simply say something like that you don't know if that would qualify them or not and maybe that should consult their physician.
I know that we sensitive people get edgy when asked questions, and often it can depend upon our mood as to how we answer too. One thing I look for is politeness, genuine interest, a real reason for wanting to know from the person(s). If these are present, I will help them as best as I can. On the other hand, if they are rude, accusitive, nasty, etc., I blow them off quicker than you can say, "BOO!".
One final thing, folks are not allowed to touch your Assistance Dog. This is ESPECIALY true of children. You will best protect yourself from this by having the dog wear some kind of markings on a garment of some kind, like a vest or a cape. It should have the color red on it. It can say something like, "Medical Assistance Dog, DO NOT PET!". This will also deter a lot of questions. Be firm, but polite, unless they get pushy, then get downright furious if you have to. WE HAVE RIGHTS! USE THEM.