**Core Concept**
Myotonic dystrophy is a type of muscular dystrophy characterized by progressive muscle wasting and myotonia. It is an autosomal dominant disorder caused by an expansion of a CTG repeat in the dystrophia myotonica-protein kinase (DMPK) gene.

**Why the Correct Answer is Right**
The DMPK gene is located on chromosome 19. The expansion of the CTG repeat in this gene leads to the production of a toxic RNA transcript that disrupts normal muscle function, causing myotonia and muscle wasting. This is a classic example of a trinucleotide repeat disorder, where the expansion of a short repeat sequence leads to a toxic gain-of-function.

**Why Each Wrong Option is Incorrect**
**Option A:** Chromosome 21 - This is incorrect because chromosome 21 is associated with Down syndrome, not myotonic dystrophy.
**Option B:** Chromosome 2 - This is incorrect because chromosome 2 is not associated with any known muscular dystrophy.
**Option C:** Chromosome 4 - This is incorrect because chromosome 4 is associated with facioscapulohumeral muscular dystrophy, not myotonic dystrophy.

**Clinical Pearl / High-Yield Fact**
Myotonic dystrophy is the most common form of muscular dystrophy that affects adults, and it is often misdiagnosed as other conditions such as Parkinson's disease or multiple sclerosis.

**Correct Answer:** C. Chromosome 19.