Most common cause of Downs syndrome is –
**Core Concept**
The question is testing the understanding of **genetic disorders**, specifically the etiology of **Down syndrome**. Down syndrome is a **chromosomal disorder** characterized by an extra copy of **chromosome 21**.
**Why the Correct Answer is Right**
The most common cause of Down syndrome is **nondisjunction** of **chromosome 21**, which occurs when a pair of chromosomes fails to separate properly during **meiosis**. This results in an extra copy of chromosome 21, leading to **trisomy 21**.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, although **translocation** can cause Down syndrome, it is not the most common cause.
**Option B:** Incorrect as **mosaicism** is another cause, but not the most frequent one.
**Option D:** Incorrect because **environmental factors** are not directly linked to the primary cause of Down syndrome.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that the risk of Down syndrome increases with **advanced maternal age**, particularly after 35 years.
**Correct Answer:** D. Nondisjunction of chromosome 21