## **Core Concept**
Down syndrome, also known as Trisomy 21, can occur due to three genetic mechanisms: nondisjunction (resulting in trisomy 21), Robertsonian translocation, and mosaicism. Robertsonian translocation involves the fusion of two acrocentric chromosomes (usually chromosomes 14, 21, 22, or 15) at the centromere, with loss of the short arms. This type of translocation can be a carrier state in parents, who are usually phenotypically normal but at increased risk of having offspring with Down syndrome.

## **Why the Correct Answer is Right**
The correct answer, . (Karyotyping of both parents), is right because when a parent is a carrier of a balanced Robertsonian translocation involving chromosome 21, there's a risk that the offspring may inherit an unbalanced set of chromosomes, leading to Down syndrome. Karyotyping both parents helps identify if either parent is a carrier of such a translocation. This is crucial for genetic counseling and assessing the risk of recurrence in future pregnancies.

## **Why Each Wrong Option is Incorrect**
- **Option A:** . (Ultrasound of the parents) - This option is incorrect because ultrasound does not provide genetic information about the parents' chromosomes or their ability to pass on genetic abnormalities to their offspring.
- **Option B:** . (Karyotyping of the baby) - While karyotyping the baby has already been done (as Down syndrome has been diagnosed), the question is about what to advise for the next pregnancy. The baby's karyotype does not directly inform us about the parents' genetic makeup or their risk of passing on similar conditions.
- **Option D:** . (Chorionic villus sampling in next pregnancy) - This option is incorrect as the immediate next step for the parents before the next pregnancy. While CVS can diagnose chromosomal abnormalities in the fetus, it is a prenatal diagnostic test done during pregnancy, not before.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that if a parent is found to carry a balanced Robertsonian translocation, the risk of having a child with Down syndrome varies depending on the specific translocation and the parent's gender. Generally, the risk is higher if the mother is the carrier. Genetic counseling is critical in such cases to discuss the risks and options.

## **Correct Answer:** . Karyotyping of both parents