Our 16.5 year old cat is a KD kitty responding well to management

Hello Hilary, I'm Dr. Deb and will do my best to help you today. I'm sorry for this concern for Possum; she sounds quite uncomfortable, doesn't she? Gently cleaning the area or applying a warm compress may help soothe her irritated skin and is definitely something that I would suggest if she were my case. Dilute cider vinegar sounds fine although she may not like the smell of it too much. Another option would be dilute Betadine or Iodine (to the color of weak tea).I probably wouldn't apply yogurt to the area since it will be incredibly messy and I'm not sure that it will bring much relief.You could smear a small amount of cortisone ointment or cream on the area but only just a very small amount. Then distract her so that she doesn't ingest the product (cats can be sensitive to many of these medications and I wouldn't want to worsen the situation).There shouldn't be enough of the ointment/cream absorbed to cause any issues for her even though she has kidney issues. Another product to consider would be Vetericyn Spray which may be available at local pet or grain stores; I know it can be purchase online but you'd have to have it shipped overnight for it to be of help. I hope this helps and that she's feeling better soon. Deb