MR Dictionary

The phenomenon where an association in the direction of a hypothesised causal relation between a risk factor and outcome is observed but is due (at least in part) to (potentially undiagnosed or precursors of) prevalent outcome influencing the risk factor. 

In the presence of valid genetic IVs for both the ‘risk factor’ and ‘outcome’, this can be tested in bidirectional MR

Bidirectional Mendelian randomization. Adapted from Richmond et al.
Figure 2.1 - Bidirectional Mendelian randomization. Adapted from Richmond et al.

References

Other terms in 'Sources of bias and limitations in MR':