**Core Concept**
Trinucleotide repeat sequences are a type of genetic mutation characterized by the expansion of a specific sequence of three nucleotides (e.g., CAG) in a gene. This expansion can lead to various neurological and psychiatric disorders. The repeats can be unstable, leading to an increase in the number of repeats with each generation, which can result in a range of disease manifestations.

**Why the Correct Answer is Right**
Trinucleotide repeat disorders, such as Huntington's disease and Spinocerebellar ataxia, are caused by the expansion of CAG repeats in specific genes. This expansion leads to the formation of a toxic protein that causes neuronal damage and degeneration. In contrast, the correct answer is associated with a different type of genetic mutation.

**Why Each Wrong Option is Incorrect**
**Option A:** Huntington's disease is indeed associated with trinucleotide repeat sequences, specifically CAG repeats in the Huntingtin gene.
**Option B:** Spinocerebellar ataxia is also associated with trinucleotide repeat sequences, such as CAG repeats in the ATXN3 gene.
**Option C:** Fragile X syndrome is associated with a trinucleotide repeat sequence, specifically CGG repeats in the FMR1 gene.

**Clinical Pearl / High-Yield Fact**
Trinucleotide repeat disorders often present with a range of neurological symptoms, including ataxia, chorea, and cognitive decline. It is essential to consider the possibility of a trinucleotide repeat disorder in patients with a family history of similar symptoms.

**Correct Answer: D.**