Sorry if this is gross!
In a nutshell, she has a scabby layer matted into her fur on her back, but no bald spots.
She is about 10 years old, short hair, and seems healthy and active. She eats the indoor cat food and uses feline pine litter. She is an indoor cat. We have a second cat who is a little bit younger but doesn't appear to have this skin problem. No fleas that I can notice on either, and I think we would notice.
The other day I was petting her and noticed that she seemed to have some scabs in her fur. I looked closer and it's like she has this extra layer of scabbing - salt and pepper looking - matted above her skin on her back by her tail, which seems to be normal looking underneath the crust. She doesn't appear to be losing fur, but will shed if I groom her with the groom brush (which she seems to enjoy), and when I did that, her skin looked a bit reddened afterwards, so I stopped. The crust smells kind of bad. I am so grossed out. I think we are going to buy some gloves tomorrow and give her a bath, because I am wondering if she is just getting old and not grooming as well... even though I see her grooming all the time??