Step 1: Minimal brain dysfunction describes children of near-normal intelligence who nonetheless struggle with attention, behaviour and specific scholastic skills.
Step 2: The hallmark learning problem of this syndrome is dyslexia, where the child cannot smoothly combine what is heard with what is seen to read fluently.
Step 3: ADHD is a related attention disorder but the specific learning deficit tied to the term is dyslexia, whereas mental subnormality and Down's syndrome are conditions of global intellectual impairment rather than isolated learning disability.
\[\boxed{\text{Dyslexia}}\]