Sore on my dog’s chin no idea what it might be!

Hello - I can see what appears to be a picture of dermatitis to the lower mandible, but dermatitis simply translates 'inflammation of the skin,' which can have many causes, so might not tell you much more than you already knew.  Causes can include scratching because of an allergy, parasites (fleas, mites), pain (eg toothache), a mischaped jaw, plant stings, fly bites or damage from putting the nose up onto, or rubbing against, an unsuitable surface.  I also wondered about Ringworm, cancer (rare presentation) and various causes of folliculitis eg demodex.  There may be others.  Please show this to your vet; skin disease often seems like such a minor, superficial thing but can be difficult to get rid of and sometimes turn chronic.  Because many different diseases can present as sore, inflamed-looking skin (especially if drool is adding to the mix), it is not unusual that several vet visits are required in order to get on top of, or diagnose, it.

This does not look like a typical case of Ringworm but Ringworm can be an imposter and should go on the list; please wear gloves when handling any bald, sore area until Ringworm and any other local infectious diseases have been ruled out.