## **Core Concept**
Down's 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 underlying principle being tested here is the cytogenetic basis of Down's syndrome.

## **Why the Correct Answer is Right**
The correct answer, , refers to non-disjunction during meiosis, leading to Trisomy 21. Non-disjunction is the failure of a pair of chromosomes to separate properly during cell division, resulting in aneuploidy. In the case of Down's syndrome, this error occurs in the formation of gametes (sperm or eggs), leading to an extra chromosome 21 in every cell of the body. This is the most common cause of Down's syndrome, accounting for about 95% of cases.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Mosaicism, which involves a mixture of normal and abnormal cells, is a cause of Down's syndrome but accounts for only about 4% of cases. It does not represent the most common cause.
* **Option B:** A Robertsonian translocation, where part of chromosome 21 breaks off and attaches to another chromosome, usually chromosome 14, is another mechanism leading to Down's syndrome. However, it is less common than non-disjunction, accounting for about 1% of cases.
* **Option D:** While there are other rare genetic mechanisms, option is not specified, implying that any other option not listed here would be incorrect based on the information provided.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the risk of having a child with Down's syndrome increases with maternal age, particularly after the age of 35. This is because the risk of meiotic errors like non-disjunction increases with advancing maternal age.

## **Correct Answer: .**