Help! Chicken sick or old?

Hi, One of my RIR'ds is just sitting in the same spot for a week now. She is not blocked and is pooping fine. I have to give her water and food and she does eat and drink.
She simply will not stand and even when she tries, her wing flares out and she almost falls over.

She is roughly 4 years old. No other chickens are showing any of these symptoms.

SO my questions are:

Is she just old and dying or perhaps she has Maricks (sp) disease.
I know its hard but it might be best to cull her if she is indeed sick?
If its Maricks- how soon would other chickens show the same symptoms?

If I do cull her-- can her meat still be eaten?

Thank you all