I can understand the worry about it being Mareks. If she has Mareks, the others have already been exposed. But I would treat her at least 2 weeks for a possible B2 riboflavin deficiency. What are you using for the riboflavin? A 1/4 of a human B complex tablet daily would supply enough riboflavin with 5 mg. Riboflavin tablets or capsules would be overkill at 100 mg per capsule, but you could scrape a little off a tablet or a tiny portion of a capsule per day.
Some things that may look like Mareks are dehydration, eating moldy feed or something in the compost bed, infection pressing on nerves in the leg, or an injury. Has there been a rooster around her who might have injured her? The best way to get a diagnosis is testing and a necropsy through your state vet if she dies. There also is a blood test available through a couple of labs that you collect yourself through a trimmed toenail, and send back in.
I hope it doesn’t turn out to be Mareks, but it doesn’t mean that you will lose all of your other chickens. Have you added any new birds to your flock recently? Are there a lot of neighbors or farms nearby that have chickens?