**Core Concept**
Huntington's chorea is a neurodegenerative disorder characterized by an autosomal dominant inheritance pattern. The disease is caused by an expansion of a CAG repeat in the Huntingtin gene, leading to a toxic protein product that causes neuronal degeneration.

**Why the Correct Answer is Right**
The causative mutation in Huntington's chorea is an expansion of a CAG repeat in the Huntingtin gene. This expansion results in a polyglutamine tract in the Huntingtin protein, which is toxic to neurons and leads to the characteristic symptoms of the disease. The pathophysiology involves the formation of protein aggregates and the activation of various cellular pathways, including apoptosis and oxidative stress.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Huntington's chorea is not caused by a mutation in the PRNP gene, which is associated with prion diseases such as Creutzfeldt-Jakob disease.
**Option B:** This option is incorrect because the Huntingtin gene is located on chromosome 4, not chromosome 14.
**Option C:** This option is incorrect because the disease is caused by an expansion of a CAG repeat, not a CGG repeat, which is associated with Fragile X syndrome.

**Clinical Pearl / High-Yield Fact**
The CAG repeat expansion in Huntington's chorea is a classic example of a trinucleotide repeat disorder, which is a type of genetic mutation that involves the expansion of a short sequence of DNA.

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