Despite the name "medicated chick feed" medicated chick feed is useless at either controlling or preventing infections because medicated chick feed is not medicated nor is it an antibiotic like we humans think of medicine. Use a topical antibiotic as recommended by birdman55 to help prevent or possibly control external infections on chicks.
I am trying to understand why this biddy still had some of the yoke sack on the outside of its body. Did she need help getting out of the shell?
The yoke sack...... sack and all.... should be completely absorbed by the chick after it hatches. The yoke sack is Mother Nature's version of a brown-bag-lunch and Miss Nature's plan is for the chick to use up this food before the chick goes out into the big bad dangerous world. This is why mamma hens usually remain on the nest with their biddies for up to 72 hours after hatching is complete. This is also a good reason to avoid staggered hatches by setting eggs that haven't ALL been properly stored and stored alike, so that ALL the eggs can begin developing at the same time and rate. When you have your first hen steal her nest away from your sight, and you think that a varmint got her, only to see her rise from the dead with 12 to 15 perfect baby chicks in tow will you begin to realize just how important good incubation practices are.
The term "Day Old Chicks" is a marketing ploy used by hatcheries to evoke pictures in the customers' mind of straw hat wearing little blue eyed children watching fuzzy baby chickens cavorting in the hay mow. Sorry for the sarcasm but I'm trying to make a point here, that is other than the point that is on top of my noggin.