The vignette turns on the $timing$ and $pattern$ of developmental regression. After an initially normal start, an infant - classically a girl - begins to lose skills very early in life and develops a distinctive movement disorder of the hands. That constellation defines $Rett$ $syndrome$.
Rett results from a mutation in the X-linked $MECP2$ gene, which is why it manifests almost only in females (affected males usually do not survive). Around 6 to 18 months the child shows arrest then regression: head circumference growth slows producing acquired $microcephaly$, purposeful hand use is lost and replaced by midline $hand$-$wringing$ or hand-washing stereotypies, and spoken language and gait deteriorate.
The examiner's marginal note '$>2$ yrs / Heller' is itself the clue to the trap. $Childhood$ $disintegrative$ $disorder$ (Heller syndrome) also features regression, but only after a prolonged period of clearly normal development - at least two and usually more than three years. Since this child regresses much earlier, Heller is excluded.
Autism lacks the acquired microcephaly and hand stereotypies, and ADHD involves no loss of milestones at all.
\[\boxed{\text{Diagnosis} = \text{Rett syndrome}}\]