This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Gynaecology & Obstetrics
A woman with a baby of downs syndrome comes for preconceptional counseling. Karyotype of the affected baby shows nondisjunction. What is the chance of downs syndrome in the subsequent Pregnancy? 
Select and explain. 
A. 1%, B. 5%, C. 10%, D. 25%
A: 1%
Answer: A: 1% 
Explanation: Recurrence RiskWith nondisjunction Down syndrome, the risk in a subsequent pregnancy would be 1% or the maternal age-related risk, whichever is higher. But in translocation Down syndrome, either parent may have translocation. In such cases, the risk to the offspring is 12% if the mother is the carrier and 3% if the father is the carrier. However, in the rare case of a carrier of a 21:21 translocation, the recurrence risk is 100%.Reference: Textbook of Obstetrics; Chapter 56; Prenatal diagnosis