## **Core Concept**
Trinucleotide repeat sequences are a type of genetic mutation where a sequence of three nucleotides is repeated multiple times. This type of mutation is associated with various neurodegenerative and genetic disorders.

## **Why the Correct Answer is Right**
The correct answer, **D. Diabetes Mellitus**, is not associated with trinucleotide repeat sequences. Diabetes Mellitus is primarily caused by defects in insulin secretion or insulin action, not by trinucleotide repeat expansions.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Huntington's disease is a classic example of a disorder caused by trinucleotide repeat expansion, specifically an expansion of a CAG repeat in the huntingtin gene.
* **Option B:** Friedreich's ataxia is caused by an expansion of a GAA trinucleotide repeat in the FXN gene, leading to mitochondrial dysfunction.
* **Option C:** Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease, is caused by an expansion of a CAG repeat in the ATXN3 gene.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that trinucleotide repeat disorders often exhibit **anticipation**, where the severity of the disease increases and the age of onset decreases with successive generations. This is due to the expansion of the repeat sequence over generations.

## **Correct Answer:** D. Diabetes Mellitus