A 3-month-old boy with Down syndrome is found on karyotype to have the abnormality marked **A** in the diagram. He has characteristic features including hypotonia, epicanthal folds, and a single palmar crease. Echocardiography reveals an atrioventricular septal defect. The structure marked **A** is a Robertsonian translocation der(14;21)(q10;q10). Which of the following is the most critical next step in the management of this family?

Explanation

## Why "Perform karyotyping on both parents to determine if either is a balanced carrier" is right Translocation Down syndrome caused by a Robertsonian translocation der(14;21) accounts for 3–4% of all Down syndrome cases and carries CRITICAL recurrence risk if a parent is a balanced carrier. Unlike free trisomy 21 (95% of cases, with ~1% recurrence), translocation DS has significantly higher recurrence risk: ~10–15% if the mother is a carrier, ~1–3% if the father is a carrier, and 100% if a parent carries der(21;21). The child's karyotype (46,XX or XY,der(14;21)(q10;q10),+21) triggers mandatory parental karyotyping to identify carrier status and guide genetic counseling and prenatal diagnosis in future pregnancies. This is the single most important clinical action after diagnosis (Nelson Pediatrics 21e, Ch 98; Robbins 10e, Ch 5). ## Why each distractor is wrong - **"Initiate immediate cardiac surgery without further genetic investigation"**: While AVSD repair is essential for the child's management, it does not address the critical genetic issue of recurrence risk. Genetic counseling and parental karyotyping must occur in parallel with cardiac care. - **"Counsel the parents that recurrence risk is 1% in future pregnancies regardless of parental status"**: This is dangerously incorrect. The 1% recurrence risk applies only to de novo cases or free trisomy 21. If a parent is a balanced carrier of der(14;21), recurrence risk is 10–15% (maternal carrier) or 1–3% (paternal carrier). If a parent carries der(21;21), recurrence is 100%. - **"Perform fluorescence in situ hybridization (FISH) on the child's blood to confirm the diagnosis"**: FISH is not necessary to confirm trisomy 21 in this case; karyotype has already established the diagnosis. FISH would be redundant and delays the critical step of parental karyotyping. **High-Yield:** Translocation Down syndrome = always karyotype both parents; der(21;21) carrier parent = 100% recurrence risk; maternal t(14;21) carrier = 10–15% recurrence; paternal t(14;21) carrier = 1–3% recurrence. [cite: Nelson Pediatrics 21e Ch 98; Robbins 10e Ch 5]