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Myotonic dystrophy is associated with an expansion of the CTG repeat in the DMPK gene. This condition is a genetic disorder that falls under the category of trinucleotide repeat disorders. In myotonic dystrophy type 1, the normal range of CTG repeats is typically less than 35, but affected individuals have hundreds to thousands of repeats, which leads to the characteristic features of the disease.

The DMPK gene encodes a protein that is important for muscle function, and its dysfunction due to the repeat expansion results in the symptoms seen in myotonic dystrophy, including muscle stiffness, weakness, and myotonia (prolonged muscle contraction). The increased number of CTG repeats leads to abnormal RNA transcripts that cause a toxic effect in the muscle cells and other tissues, contributing to the clinical manifestations of the disorder.

In contrast, the other conditions listed—Duchenne muscular dystrophy, Becker muscular dystrophy, and spinal muscular atrophy—are not related to CTG repeat expansions within the DMPK gene and have different genetic causes and mechanisms.