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Content validity of the 18-item Investigator-Rated Attention-Deficit Hyperactivity Disorder (ADHD) Rating Scale IV (I-ADHD RS-IV) with adult prompts was investigated using qualitative interviews of US clinicians who had prior experience rating adults with phenylketonuria (PKU) using the I-ADHD RS-IV. Fourteen qualitative interviews were conducted to obtain key symptom experiences of adults with PKU and assessed the relevance, clarity, and administration of the I-ADHD RS-IV. Participants (n = 13, 92.9%) endorsed the inattention symptoms as key experiences by adults with PKU and endorsed the instrument as fit for purpose for adults with PKU. Participants generally reported low frequencies of occurrence for the 9 I-ADHD RS-IV hyperactivity/impulsivity items. Despite some clinicians’ concerns for the lack of patient self-awareness, the participants reported no difficulty selecting a rating on these items. This in-depth study of the content validity of the I-ADHD RS-IV provides evidence that this clinician-reported instrument captures the severity of important inattention symptoms in adults with PKU.