## **Core Concept**
Myotonic dystrophy is a genetic condition characterized by progressive muscle wasting and weakness. It is caused by an expansion of a repeated sequence in a specific gene, leading to a type of muscular dystrophy that affects various systems in the body, including the musculoskeletal, nervous, and endocrine systems.

## **Why the Correct Answer is Right**
The correct answer, **C. CTG repeat expansion in DMPK gene**, is right because myotonic dystrophy type 1 (DM1) is caused by an expansion of CTG repeats in the DMPK (dystrophia myotonica-protein kinase) gene on chromosome 19. This expansion leads to the formation of aberrant RNA that sequesters and disables various RNA-binding proteins, disrupting normal RNA processing and leading to the clinical manifestations of the disease, including myotonia, muscle weakness, and systemic features.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Incorrect because while chromosomal abnormalities can cause various muscular dystrophies, the specific genetic mechanism for myotonic dystrophy involves a repeat expansion, not a simple chromosomal deletion or translocation.
- **Option B:** Incorrect as it refers to a different genetic condition. The FRDA gene is associated with Friedreich's ataxia, a different type of neurodegenerative disorder caused by an expansion of GAA repeats.
- **Option D:** Incorrect because the GGGGCC repeat expansion in the C9ORF72 gene is associated with certain forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia, not myotonic dystrophy.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl for myotonic dystrophy is the presence of **myotonia**, which is a delayed relaxation of muscles after contraction. This can be tested by asking the patient to grip and then release their hand quickly; in myotonic dystrophy, the hand will take several seconds to release. This clinical feature is a hallmark of the disease.

## **Correct Answer:** C. CTG repeat expansion in DMPK gene