Is there any chance you can upload a photo (or, better yet a video to YouTube/ rumble/ etc and post a link) of the baby standing/ walking?
Is an avain vet an option for you?
The fact that there appears to be bruising makes me feel like it’s more likely an injury. Esp because I also see the “cut”
Is this baby w a broody, or in a brooder?
If not w mama, what is the surface of/ footing in your brooder like?
Is the baby able to get around at all?
Eating/ drinking?
I saw your recent reply- are you also noticing any heat in the affected joint?
I’d start w an Epsom salt soak. Honestly for a chick that small, I’d probably dissolve the Epsom salt into a bowl of warm water, put a towel on the “treatment area”, fold up a paper towel, hold around the “hock”, and re dip, re apply every few minutes with a goal of hitting 10-15m at the minimum.
I would also add vitamin B and E to the baby’s feed/ water.
Durvet B concentrate (it’s an injectable for cows but can be administered orally, over treats, or in water, if the baby is drinking.... I would need to look up dosage...
Amazon or most local feed stores carry it)
I was using 1ml over soldier fly larvae for a 4 week old duckling last time I did the math on it.... but your baby is smaller so let us know if we need to look up the dosage.
Or.
Nutritional yeast on the feed can help as a vitamin B supplement, or 1/4 tablet B Complex crushed plus 400IU Vitamin E daily along with a bit of egg (I would cook a yolk or two and add the vitamins and offer first to the baby in question- and let the others finish off any leftovers).
The B Complex is for capillary health and leg support, and the E is for neural support.
This thread might help you as well
https://www.backyardchickens.com/articles/leg-foot-and-toe-issues-in-poultry-of-all-ages.68093/
@Eggcessive @Miss Lydia
....anything you can think of that I’ve missed? Or that I’m wrong about?