MAPT and Alzheimer disease: Here, we tested the following hypotheses: (1) ADHD‐PRS is associated with worsening cognitive function in patients with MCI and AD; (2) ADHD‐PRS correlates with elevated biomarkers of AD pathology within this population, including CSF levels of Aβ and p‐tau, and fluorodeoxyglucose positron emission tomography ([18F]FDG‐PET); and (3) lower cognitive function linked with ADHD‐PRS is mediated by brain glucose hypometabolism.