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Myotonic Dystrophy Type 1 — SCE Neurology MCQ

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HardNeurogeneticsMyotonic Dystrophy Type 1SCE Neurology

A woman has grip myotonia, distal weakness, ptosis, frontal balding, dysphagia and first-degree heart block. Which molecular defect causes the disorder?

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Correct answer: D — An expanded CTG trinucleotide repeat in the 3′ untranslated region of DMPK

The best answer is “An expanded CTG trinucleotide repeat in the 3′ untranslated region of DMPK”. Myotonic dystrophy type 1 results from a DMPK CTG-repeat expansion with anticipation and multisystem skeletal-muscle, cardiac and endocrine disease. “A CAG repeat expansion in HTT” remains a plausible alternative in a different presentation, but it does not fit the decisive finding or management threshold here. “A GAA repeat expansion in FXN” remains a plausible alternative in a different presentation, but it does not fit the decisive finding or management threshold here. “A deletion of exons in DMD” remains a plausible alternative in a different presentation, but it does not fit the decisive finding or management threshold here. “A pathogenic mitochondrial DNA deletion confined to muscle” remains a plausible alternative in a different presentation, but it does not fit the decisive finding or management threshold here.

Reference: GeneReviews: Myotonic Dystrophy Type 1: https://www.ncbi.nlm.nih.gov/books/NBK1165/