Found out my dog has mange?

so we adopted a dog from our local shelter at the end of October and I recently noticed she had a little red patch of missing hair no bigger then a dime on her front right leg so I took her in for a check up and they said it was demodectic mange, she is a year old and I have been doing some research online but her... show more so we adopted a dog from our local shelter at the end of October and I recently noticed she had a little red patch of missing hair no bigger then a dime on her front right leg so I took her in for a check up and they said it was demodectic mange, she is a year old and I have been doing some research online but her leg doesn't look anything like what the symptoms describe or any of the pictures and so far I have just read it is uncommon in adult dogs to just flare up and it is usually an underlying immune deficiency, the spot is also only located in that one small spot on the front of her right leg like a lick sore or something she doesn't bother it though never itches or bites at it or anything and it hasn't spread at all, they said they did a skin scraping and confirmed it but I dont know does anyone have any advice or has had a similar experience, thank you for your help
Update: the vet wanted to do ivermectin and some other stuff but if I leave it alone it will heal up within a few weeks
Update 2: this is a 4.5 lb chihuahua if that makes any difference as well thank you all for you comments they are all very helpful
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