Down syndrome trisomy 21

A newborn boy delivered at 39 weeks to a 38-year-old primigravida presents with upslanting palpebral fissures, epicanthic folds, flat nasal bridge, protruding tongue, a single palmar crease, clinodactyly of the 5th finger, and wide sandal gap between the 1st and 2nd toes. Echocardiography reveals a complete atrioventricular septal defect. Karyotype analysis shows the chromosomal abnormality marked as **C** in the diagram. Which of the following best describes the karyotype finding and its most common etiology?

A neonate born to a 38-year-old primigravida at 38 weeks gestation presents with characteristic dysmorphic features including upslanting palpebral fissures, epicanthal folds, a flat nasal bridge, and hypotonia. On examination of the palms, the structure marked **A** is noted bilaterally. A karyotype confirms trisomy 21. Which of the following best describes the clinical significance of the finding marked **A** in Down syndrome?