Urgent! 6 week old chick with no leg strenght

[Mac84]

I have a 6 week old cochin bantam chick that started losing strenght on one leg a couple of days ago. She seems to be struggling more and more. The leg doesnt seem broken and she is able to move them to get water and food but spends all day laying down with the leg to the side.
Otherwise eating and drinking and poops are normal.
I am worried about mareks even thou I was told the chicks were vaccinated.
I just bought some vitamin E with Selenium that I would like to try. How much do I give her?
What else can I try?
I appreciate the help.

 

[Eggcessive]

I don’t think that Mareks even shows up ever before the age of 5 weeks old, and 6 weeks would be still pretty rare. She could have injured her tendon, but she might also be showing signs of a leg bone disorder such as varus deformity. If it is that, there is no treatment. I would also check for a slipped tendon. That is when the hock joint appears flat. Here is a video of fixing one: