A 32-year-old woman delivers a baby at term with flat facies, upslanting palpebral fissures, epicanthal folds, a single palmar crease, and hypotonia. Echocardiography shows an ostium primum atrial septal defect. Karyotyping reveals 47,XX,+21. The most likely mechanism responsible for this child's aneuploidy is:

• A Postzygotic mitotic nondisjunction producing mosaicism
• B Nondisjunction during paternal meiosis II
• C Robertsonian translocation between chromosomes 14 and 21 in one parent
• D Nondisjunction during maternal meiosis I ✓

Explanation

About 95% of full trisomy 21 cases result from meiotic nondisjunction, and approximately 90% of these occur in the mother, mostly in meiosis I, with risk rising sharply after age 35. D Robertsonian translocation accounts for only about 4% of cases and would show 46 chromosomes on karyotype, so it is excluded by the reported 47-count. Mosaicism produces a milder phenotype. The presence of an endocardial cushion defect (ostium primum ASD) is the classic cardiac lesion of Down syndrome but does not alter the cytogenetic mechanism.

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.