A newborn has hypotonia, flat facies, single palmar crease, and a cardiac defect. Karyotype shows 47 chromosomes. The MOST common mechanism causing this syndrome is:

• A Translocation between chromosome 14 and 21 (Robertsonian)
• B Mosaicism arising from mitotic nondisjunction
• C Isochromosome formation
• D Meiotic nondisjunction producing trisomy 21 (free trisomy) ✓

Explanation

Down syndrome (trisomy 21) is most commonly caused (>95% of cases) by meiotic nondisjunction during oogenesis (usually meiosis I), producing an egg with two copies of chromosome 21. Robertsonian translocation accounts for ~4% and mosaicism ~1%. The risk of meiotic nondisjunction increases dramatically with maternal age. Free trisomy 21 carries a low recurrence risk, unlike translocation DS where recurrence risk depends on carrier parent.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.