CRP and obesity due to melanocortin 4 receptor deficiency: This relies on the assumption that the instrument‐exposure effect is the same in the diseased compared with the total population, which may not be correct; for example, a recent analysis found that the association between C‐reactive protein (CRP) SNPs and measured CRP differed between cases and controls for obesity and type 2 diabetes.(102) A full review of methods to identify and correct for collider bias in MR of disease progression is available elsewhere.(103)