Step 1: Minimal brain dysfunction is a neurodevelopmental disorder marked by immaturity in the control of activity, emotions and behaviour, with specific learning disabilities in reading, writing and mathematics.
Step 2: A core clinical feature of minimal brain dysfunction is dyslexia, a specific reading disability reflecting impaired blending of auditory and visual functions in language.
Step 3: ADHD overlaps clinically but the classic associated learning disability described under this term is dyslexia, while mental subnormality and Down's syndrome are global intellectual conditions, not the specific learning-disability picture. Hence dyslexia is the answer.