## **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 intellectual disability, delayed speech, and distinct physical characteristics. The causes of Down syndrome are primarily genetic.

## **Why the Correct Answer is Right**
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. Nondisjunction is a failure of chromosome pairs to separate properly during cell division, leading to abnormal numbers of chromosomes in the offspring.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while Robertsonian translocation can cause Down syndrome, it is much less common than nondisjunction, accounting for about 4% of cases.
* **Option B:** This option is incorrect as mosaicism, which involves only some cells having an extra copy of chromosome 21, is another cause but occurs less frequently than nondisjunction, responsible for about 1% of cases.
* **Option D:** This option is incorrect because other genetic variations can lead to Down syndrome-like features but are not the most common cause of classical Down syndrome.

## **Clinical Pearl / High-Yield Fact**
A key point to remember 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 the non-invasive prenatal test (NIPT) and invasive diagnostic tests like amniocentesis, can detect Down syndrome during pregnancy.

## **Correct Answer:** . Nondisjunction.