**Core Concept:** Huntington disease is an autosomal dominant neurodegenerative disorder caused by an expansion of CAG trinucleotide repeats within the HTT gene. The number of repeats determines the age of disease onset and severity.

**Why the Correct Answer is Right:** In Huntington disease, the CAG trinucleotide repeats expand from the normal range (5-35) to more than 35 repeats. This expansion leads to the production of an abnormally long polyglutamine tract in the huntingtin protein. This misfolded protein causes neuronal dysfunction and death, particularly affecting medium spiny neurons in the striatum, leading to the characteristic motor, cognitive, and psychiatric symptoms seen in the disease.

**Why Each Wrong Option is Incorrect:**

A. Huntington disease involves a different type of genetic mutation: trinucleotide repeat expansion, not just a single nucleotide mutation like option B suggests.

B. Single nucleotide mutations, not trinucleotide repeat expansions, are associated with diseases like cystic fibrosis or sickle cell anemia.

C. Although autosomal dominant inheritance is a feature of Huntington disease, the correct term for the genetic mutation is trinucleotide repeat expansion, not just autosomal dominant inheritance like option D suggests.

**Clinical Pearl:** The CAG repeat size is inversely correlated with the age of disease onset, meaning larger expansions lead to earlier disease onset. This genetic test is crucial for genetic counseling and disease prediction in family members of affected individuals.

**Correct Answer:** C. Huntington disease involves trinucleotide repeat expansion within the HTT gene, leading to the production of an abnormally long polyglutamine tract and subsequent neuronal dysfunction and death.