A sudden large area of ​​redness on the inside of a dog's hind legs may be a skin disease caused by fungal bacterial infection, dermatitis, eczema, etc. The cause needs to be confirmed based on the specific situation of the dog, and then treated accordingly. If it is a fungal infection, you can use a pet-specific fungal spray. If it is dermatitis and eczema, you can use an anti-inflammatory ointment.
Tips:
You should bathe your dog regularly and deworm them. At the same time, you should keep the environment clean and sanitary, and do not let your dog lie down in wet places.