Which disorder is characterized by progressive neurological deterioration?

Huntington's disease is indeed characterized by progressive neurological deterioration. This genetic disorder is caused by a mutation in the HTT gene, leading to the production of a toxic protein that gradually damages brain cells. The most notable features of Huntington's disease include chorea (involuntary movements), cognitive decline, and emotional disturbances, all of which progressively worsen over time.

In contrast, the other disorders listed do not primarily involve progressive neurological deterioration. For example, Fragile X syndrome is a genetic condition leading to intellectual disabilities but does not progressively worsen in the same manner as Huntington's. Cystic fibrosis primarily affects the respiratory and digestive systems due to thickened mucus production, while Down syndrome is a chromosomal condition linked to developmental delays but not progressive neurological decline. Thus, Huntington's disease stands out as the condition best represented by the characteristic of progressive neurological deterioration.