It takes at least 3 weeks after exposure to Mareks for symptoms to show up in any chicken or baby chick. And 3 weeks old would still be rare for a chick with Mareks to have those symptoms. Usually Mareks symptoms would normally occur between 5 weeks to 7 months old if exposed.
I might be imagining it, but the left leg seems most affected in the video. In the pic below, the right leg has valgus deformity, and varus or valgus deformities are common in all poultry. They can affect one leg or both, and when they affect both legs, the hock joints can appear eith knock kneed in valgus or bowlegged in varus deformity. However, I would still give the vitamins and hope it it just an injury or deficiency. Here is some info on varus valgus deformities, and another link about how to make a chick chair to get her upright:
https://europepmc.org/backend/ptpmcrender.fcgi?accid=PMC1790586&blobtype=pdf
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/
I might be imagining it, but the left leg seems most affected in the video. In the pic below, the right leg has valgus deformity, and varus or valgus deformities are common in all poultry. They can affect one leg or both, and when they affect both legs, the hock joints can appear eith knock kneed in valgus or bowlegged in varus deformity. However, I would still give the vitamins and hope it it just an injury or deficiency. Here is some info on varus valgus deformities, and another link about how to make a chick chair to get her upright:
https://europepmc.org/backend/ptpmcrender.fcgi?accid=PMC1790586&blobtype=pdf
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/