Huntington Disease — Autosomal Dominant Inheritance

A 42-year-old man presents to the neurogenetics clinic with progressive chorea, dysarthria, and irritability beginning at age 40. His father died at age 55 with similar symptoms, and his paternal grandfather was institutionalized for "dance-like" movements. The family pedigree (marked **B** in the diagram) shows both males and females affected across every generation with clear father-to-son transmission and approximately half of offspring affected in each generation, with no skipping of generations. Which of the following molecular findings at the location marked **B** would be most consistent with the diagnosis of Huntington disease?