Sounds stupid but... a single A-minus?

Hi guys, this is my first post here so please don't be cruel even though this is probably a stupid question.

I'm a junior and I'm on track to get either an A- or a B+ this year in an AP-like bio course. Previously, I had a perfect 4.0 for all of high school. I hope to apply to some T20 schools. Is this a minor problem for my chances, or a complete nonissue?

I don't want responses like "there's people that are dying" or whatever, I know this isn't a disaster. I just want genuine thoughts.

TIA