Excuse me, is this a dog skin disease? If yes, what medicine should I use?

Fungal infections or mites are very easy to treat. My dog used to be like this, but now it is more troublesome to do it in winter: first, cut off the hair of the symptomatic part, then buy: potassium permanganate, vitamin C, vitamin B, dilute potassium permanganate and rub the symptomatic part for two minutes, then soak the vitamin C tablet in water to wash potassium permanganate, otherwise it will burn the dog's skin, once a day, and then apply Piyanping to try not to let it lick. Take a bath once a week. Every bath must be dried as soon as possible. It won't take more than ten days. You can try, but it's a little troublesome in winter.