Wilson disease stands apart from the other four conditions because it is the only one with a treatment that genuinely alters and can reverse the disease course. The defect lies in the $ATP7B$ copper-transporting ATPase, leading to toxic copper deposition in the liver and the lenticular nuclei. The therapeutic principle is simple: remove and block copper. Chelators such as D-penicillamine and trientine mobilise stored copper for urinary excretion, while oral zinc induces metallothionein in enterocytes to trap dietary copper and shed it in stool. Begun early, this regimen prevents progression and can reverse Kayser-Fleischer rings and neurological signs; transplantation cures the underlying hepatic defect in fulminant cases.
Contrast this with the alternatives. PKAN and PLA2G6-associated neurodegeneration are inherited NBIA syndromes with relentless progression and only symptomatic care for dystonia and spasticity. PML, a JC-virus leukoencephalopathy, responds only to immune reconstitution, not to a specific cure. Hence the unique, disease-modifying option is copper-targeted therapy.
\[\boxed{\text{Wilson disease}}\]