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If you are sure this isnt a nipple or a belly button (these mistakes happen more often than one might think!) please make an appointment with your vet, who can diagnose and prescribe appropriately. A bald patch may be mites, fungal or bacterial skin infections, or piggy self barbering as a result of pain, or urine scald or just a dry irritated skin patch if their tummy drags on the floor or they sleep in their wee- only a vet can do the diagnostic tests to check!