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PD RBD – Melatonin Preferred — SCE Neurology MCQ

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HardSleep DisordersPD RBD – Melatonin PreferredSCE Neurology

A 62-year-old has a decade of progressive sensory neuronopathy, cerebellar ataxia, bilateral vestibular areflexia and chronic spasmodic cough. Nerve-conduction studies show absent sensory responses; a dominant family history is absent. Which genetic investigation has the highest diagnostic yield?

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Correct answer: C — Test specifically for biallelic pathogenic RFC1 AAGGG repeat expansions

The combination of cerebellar ataxia, neuropathy or sensory-neuronopathy, bilateral vestibular areflexia and chronic cough is the RFC1 CANVAS spectrum. The common pathogenic intronic AAGGG repeat expansion requires a repeat-expansion assay and is commonly missed by routine exome or sequence-only panels. HTT testing addresses Huntington disease, while FXN testing is appropriate for Friedreich ataxia but does not fit the late-onset CANVAS phenotype.

Reference: GeneReviews: RFC1 CANVAS and spectrum disorder (Current specialist genetic reference, accessed July 2026): https://www.ncbi.nlm.nih.gov/books/NBK564656/