A newborn presents with hypotonia, upslanting palpebral fissures, a single palmar crease, and a congenital heart defect. Cytogenetic analysis confirms trisomy 21. Which mechanism most commonly leads to this chromosomal abnormality?
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Correct answer: Meiotic nondisjunction
Most cases of trisomy 21 are due to meiotic nondisjunction, particularly during maternal meiosis I. It results in an extra copy of chromosome 21. Other mechanisms, like Robertsonian translocation, account for a smaller proportion of cases.
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