A newborn has hypotonia, Brushfield spots, endocardial cushion defect, and simian crease. Karyotype shows trisomy 21. The risk of this condition increases with maternal age primarily because of:
- A Increased paternal germ cell mutations in older fathers
- B Meiosis I non-disjunction in the mother's oocytes that have been arrested at prophase I since birth ✓
- C Decreased DNA repair capacity in embryonic cells
- D Increased environmental mutagen exposure over time
Correct answer: B. Meiosis I non-disjunction in the mother's oocytes that have been arrested at prophase I since birth
Explanation
Trisomy 21 (Down syndrome) results predominantly (~95%) from non-disjunction during maternal meiosis I. Oocytes are arrested at the diplotene stage of prophase I from birth until ovulation (decades later). This extended arrest allows cohesin degradation, increasing susceptibility to meiotic errors with advancing maternal age. Approximately 80% of non-disjunction events are maternal. Paternal age plays a minor role in trisomies.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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