A 30-year-old woman with myotonic dystrophy (CTG expansion in DMPK, 400 repeats) is pregnant. Her newborn has generalized hypotonia, respiratory insufficiency requiring ventilation, facial diplegia, and clubfeet. Which statement correctly predicts the relationship between transmitting parent and severity in this family?
- A Congenital severe disease occurs almost exclusively when the mother transmits the expanded allele ✓
- B Congenital severe disease occurs almost exclusively when the father transmits the expanded allele
- C Severity depends only on repeat size, not on which parent transmits it
- D Maternal transmission produces milder disease than paternal transmission of identical repeat lengths
Correct answer: A. Congenital severe disease occurs almost exclusively when the mother transmits the expanded allele
Explanation
Myotonic dystrophy shows anticipation through progressive CTG expansion. For the severe congenital form, with marked hypotonia and neonatal respiratory failure, transmission is almost always maternal, thought to reflect additional effects of the myotonic intrauterine environment on the fetus. This contrasts with Huntington disease, where large expansions arise nearly exclusively through paternal transmission, making option B a tempting but incorrect distractor.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.