I think you can pretty much rule out Mareks since it is too young to show symptoms.
It could be a vitamin deficiency, The symptoms sound like a vitamin E deficiency but it could have other vitamin deficiencies as well. Do you have an "Save a Chick" on hand? I would separated this chick to be on the safe side. Some respiratory diseases are passed vertically so you want to rule out a disease before exposing your other chicks.
It could be a form of pneumonia. Sometimes chicks can aspirate liquid while hatching or later learning how to drink at first. Aspergillosis is also common in brooders ( so called brooder pneumonia ), but the chick is so young I would not think it would have time to develop symptoms from fungus.
Trisha
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