ready to go get something to fix this. what should i pick up?

my girl is like 4 weeks into flowering and the yellowing just started late yesterday and now today it got a little worse. ive been giving it N in its nutes on regular schedual the ph is straight. or is it about the time it starts to yellow..(first indoor grow) ..
what should i go pick up at the store right now. wanna get there while im still high im ready to go.
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Looks okay to me. Some calcium or magnesium might help, dependant upon the contents of your other nutrients and what water you're using.

Personally I wouldn't even worry. What's one leaf, when it's too far from the light (from the looks of things) to be any use anyway? Plus you're well into flowering now and all the other leaves look fine.

I would've stopped giving it nitrogen-rich nutrients two weeks ago and started on a more balanced nutrient solution. That's me though; live and learn!

Good luck.

Totally alright man, if you try to fix that you will have too much N by the end of flowering, just let it do its thing.