**Core Concept**
Alzheimer's disease is primarily associated with amyloid-beta plaques and tau protein tangles in the brain, whereas trinucleotide repeat disorders involve the expansion of specific CAG or CGG repeats in genes leading to toxic protein products.

**Why the Correct Answer is Right**
Alzheimer's disease is not caused by the overexpression of a trinucleotide repeat. Instead, it is characterized by the accumulation of amyloid-beta peptides and hyperphosphorylated tau protein, leading to neuronal damage and death. This disease is caused by a combination of genetic and environmental factors, including mutations in the APP, PSEN1, and PSEN2 genes. In contrast, trinucleotide repeat disorders, such as Huntington's disease and spinocerebellar ataxia type 2, are caused by the expansion of CAG repeats in the Huntingtin gene and ATXN2 gene, respectively.

**Why Each Wrong Option is Incorrect**
**Option B:** Fragile X syndrome is indeed caused by the overexpression of a trinucleotide repeat, specifically a CGG repeat expansion in the FMR1 gene, leading to the silencing of the gene and subsequent deficiency of the fragile X mental retardation protein (FMRP).
**Option C:** Huntington disease is caused by the expansion of a CAG repeat in the Huntingtin gene, leading to the production of a toxic protein that causes neuronal damage and death.
**Option D:** Spinocerebellar ataxia type 2 is caused by the expansion of a CAG repeat in the ATXN2 gene, leading to the production of a toxic protein that causes neuronal damage and death.

**Clinical Pearl / High-Yield Fact**
Trinucleotide repeat disorders are a group of neurodegenerative diseases caused by the expansion of CAG or CGG repeats in specific genes, leading to the production of toxic protein products that cause neuronal damage and death. These disorders are characterized by progressive neurological decline and often have a dominant pattern of inheritance.

**✓ Correct Answer: A. Alzheimer's disease**