Did the vet check for scabies? The problem near the anus could be due to worms. You should also look online for information on Candida infections. Did the cat have any antibiotics, recently? If you do not get any answers, maybe you should take the cat to a different vet for a second opinion. Additionally, fleas can cause scabs, but is certainly seems the vet would have seen signs of this. Fleas cause scabby areas, and also what appears to be "dirt" under the fur, which is actually minute scabs and also excrement from the fleas. Sometimes a flea can be spotted, but not always. Part the fur and check thoroughly if fleas are suspected. If the cat does have fleas, then a bath in water with dish soap will kill them effectively. Follow with a second bath in a week or two. Use a flea comb around the face to capture any fleas that have climbed to safety in that area. Dish soap baths will kill fleas without causing harm from chemical treatments.