his $37-$year$-$old man presents to the psychiatry emergency room for inappropriate behavior and confusion. He works as a janitor and has had reasonably good work attendance. His coworkers say that he has appeared $$fidgety$$ for several years. They specifically mention jerky movements that seem to affect his entire body more recently. His mother is alive and well, although his father died at age $28$ in an auto accident.

On examination, he is alert but easily distracted. His speech is fluent without paraphasias but is noted to be tangential$.$ He has trouble with spelling the word $$world$$ backwards and serial seven$$s$,$ but recalls three objects at $3$ minutes. His constructions are good. When he walks, there is a lot of distal hand movement, and his balance is precarious, although he can stand with both feet together. His reflexes are increased bilaterally, and there is bilateral ankle clonus. A urine drug screen is negative.

Most likely diagnosis: Huntington disease.

Next diagnostic step: Genetic counseling and genetic testing for Huntington disease. Review the history very carefully with patient and his relatives and assess medications$$either illicit or licit that could be responsible.

Molecular or genetic basis: Repeat CAG triplets present in a gene called huntingtin located on chromosome $4$p$16\mathrm{.}3.$ Repeat lengths greater than $40$ are nearly always associated with clinical Huntington$$s disease. Confirmed Huntington$$s disease.

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