## **Core Concept**
Down syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. This condition leads to various physical and developmental delays. The core concept here involves understanding the genetic basis of Down syndrome.

## **Why the Correct Answer is Right**
The correct answer, , is associated with Down syndrome because the most common cause of Down syndrome is nondisjunction during meiosis, resulting in an extra copy of chromosome 21 in every cell of the body, known as trisomy 21. This occurs in about 95% of cases. The mechanism involves a failure of chromosome pairs to separate properly during cell division.

## **Why Each Wrong Option is Incorrect**
* **Option A:** - This option is incorrect because while Robertsonian translocation (involving chromosome 21) is indeed associated with Down syndrome, it is less common, occurring in about 4% of cases.
* **Option B:** - This option is incorrect as it does not directly relate to a known common cause of Down syndrome.
* **Option D:** - This option is incorrect because Mosaicism (having a mixture of normal and trisomy 21 cells) is a cause of Down syndrome but is less common than trisomy 21, occurring in about 1% of cases.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the risk of having a child with Down syndrome increases with maternal age, particularly after the age of 35. Screening tests, such as non-invasive prenatal testing (NIPT), ultrasound markers, and diagnostic tests like amniocentesis, can help detect Down syndrome during pregnancy.

## **Correct Answer:** . Trisomy 21