Myotonic dystrophy:

Ans. A. CTG trinucleotide repeatsExplanationMyotonic dystrophy is an autosomal dominant multisystem disorder associated with skeletal muscle weakness, cataracts, endocrinopathy, and cardiomyopathy. The disease is caused by expansions of CTG triplet repeats in the 3'-noncoding region of the myotonic dystrophy protein kinase (DMPK) gene.