**Core Concept**
Huntington's chorea is a neurodegenerative disorder caused by an expansion of a CAG repeat in the Huntingtin gene, leading to the production of an abnormal protein called huntingtin. This mutation causes a toxic effect on neurons, particularly in the basal ganglia, resulting in motor symptoms and cognitive decline.

**Why the Correct Answer is Right**
The causative mutation in Huntington's chorea is an expansion of a CAG repeat in the Huntingtin gene, which leads to the production of an abnormally long polyglutamine tract in the huntingtin protein. This polyglutamine expansion causes a conformational change in the protein, leading to the formation of aggregates that are toxic to neurons. The expansion of the CAG repeat is a gain-of-function mutation, where the abnormal protein causes a toxic effect on neurons, leading to the symptoms of Huntington's chorea.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the mutation in Huntington's chorea is not related to a point mutation or a frameshift mutation in the Huntingtin gene.

**Option B:** This option is incorrect because the mutation in Huntington's chorea is not related to a deletion or duplication of the Huntingtin gene.

**Option C:** This option is incorrect because the mutation in Huntington's chorea is not related to a point mutation or a frameshift mutation in the Huntingtin gene, but rather an expansion of a CAG repeat.

**Clinical Pearl / High-Yield Fact**
The expansion of a CAG repeat in the Huntingtin gene is a classic example of a gain-of-function mutation, where the abnormal protein causes a toxic effect on neurons, leading to the symptoms of Huntington's chorea. This is a key concept to remember in neurodegenerative disorders.

**Correct Answer:** A. Expansion of a CAG repeat in the Huntingtin gene.