I was thinking a venous abnormality of some kind. It wouldn't be trauma related, as its in both legs.
My best opinion? This chick has a birth defect of some kind that is preventing good blood flow and normal development. As your other chickens of similar age, and different ages, are doing fine on your laying rations, I would say this bird had something wrong with it internally at hatch.
I butchered a hen a while back that had never layed and whose comb hadn't developed well. She had one lung that was smaller and more pale than the other. She probably had other defects that I did not discover because I hadn't examined her other organs closely.