Definitely ignore single studies, no matter how prestigious the journal or numerous the citations.

Straight-up replications are rare, but if a finding is real, other PIs will partially replicate and build upon it, typically as a smaller step in a related study. (E.g., a new finding about memory comes out, my field is emotion, I might do a new study looking at how emotion and your memory finding interact.)

If the effect is replicable, it will end up used in other studies (subject to randomness and the file drawer effect, anyway). But if an effect is rarely mentioned in the literature afterwards...run far, FAR away, and don't base your research off it.

A good advisor will be able to warn you off lost causes like this.