DEAR MISS MANNERS: I have naturally red cheeks, often referred to as rosacea. I also tend to blush easily. I am at a loss for an appropriate response when friends and acquaintances ask, "Why is your face so red?"
GENTLE READER: Try looking down at the ground and saying, "You're embarrassing me."
Notice that you will not necessarily have claimed to be blushing if you are not. You could say this in response to any nosy question. But it is a particularly apt way of dealing with this particular unfortunate remark.