10 month old Barred Rock pullet. Home hatched from an incubator, so no vaccines. Yesterday, I saw one of my birds kind of limping/having weakness on her left leg yesterday. She could walk fine and was drinking so I assumed that she might of fallen off the roost and sprained something. She's near the bottom of the pecking order. This morning, she was sitting in the coop and when I stood her up, she stood crouched. When she tried to walk, she barely could and she used her wings a lot for balance. I separated her and left her with vitamin/electrolyte water and food for the time being. I remember last week my grandma commenting that the bird was a bit light. She is a bit on the skinny side for her breed. Right now she is very alert but still cannot walk right. She is not lethargic and she is even trying to scratch the bottom of her enclosure. When I took her out to walk, she tried. She can move her legs but they are weak and her toes kind of rolled under her foot. She ate some feed and meal worms out of my hand, the meal worms with vigor. I noticed that her poop is very dark in color, very smelly, and has some undigested kitchen scraps that I had fed this morning/left with her (cut up mango peels). Her crop feels normal and not squishy so I think I can rule out sour crop. I'm thinking that the best case scenario that this is a vitamin deficiency that should correct itself with the vitamin water. Worst case is that it is Marek's. I asked my county's 4-H educator about the situation and she said that she highly doubts that it's Marek's since she hasn't heard of any cases in the county recently and no one else in my flock has symptoms. I'm taking her to the vet tomorrow along with a stool sample. I think that it might be parasites that are causing are to be light and to possibly have a vitamin deficiency.