All the following diseases are associated with triplet and other repeat sequences except
**Core Concept**
The question is testing the association between triplet repeat expansions and various diseases. Triplet repeat expansions occur when a sequence of three nucleotides is repeated abnormally many times in a gene, leading to genetic instability and potentially causing disease. This phenomenon is linked to several neurodegenerative and hereditary disorders.
**Why the Correct Answer is Right**
The correct answer is associated with triplet repeat expansions, which are a common cause of genetic disorders. This mechanism is responsible for diseases such as Huntington's disease, myotonic dystrophy, and spinocerebellar ataxia. The expansion of these repeats disrupts normal gene function, leading to cellular toxicity and tissue damage.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Huntington's disease is indeed associated with a triplet repeat expansion in the Huntingtin gene. The CAG repeat expansion in this gene is responsible for the pathogenesis of the disease.
**Option B:** This option is incorrect because myotonic dystrophy type 1 is caused by a CTG repeat expansion in the DMPK gene. This repeat expansion leads to the formation of toxic RNA molecules that disrupt normal muscle function.
**Option C:** This option is incorrect because spinocerebellar ataxia type 1 is caused by a CAG repeat expansion in the ATXN1 gene. This repeat expansion leads to the formation of toxic protein molecules that disrupt normal neuronal function.
**Clinical Pearl / High-Yield Fact**
The expansion of triplet repeat sequences is a common mechanism underlying various hereditary disorders. It's essential to recognize the association between these repeats and specific diseases to provide accurate genetic counseling and diagnosis.
**Correct Answer:** A.