Which is the most common chromosomal anomaly seen?
**Core Concept**
The question pertains to chromosomal anomalies, specifically the most common one. Chromosomal anomalies occur due to errors during meiosis or mitosis, leading to changes in the number or structure of chromosomes. These anomalies can result in various genetic disorders.
**Why the Correct Answer is Right**
Trisomy 21, also known as Down syndrome, is the most common chromosomal anomaly, occurring in approximately 1 in every 700 births. This condition arises due to an extra copy of chromosome 21, resulting from nondisjunction during meiosis. The extra genetic material disrupts the normal functioning of cells, leading to characteristic physical features, intellectual disability, and increased risk of certain medical conditions.
**Why Each Wrong Option is Incorrect**
**Option A:** Edwards syndrome (Trisomy 18) is a less common chromosomal anomaly, occurring in approximately 1 in 5,000 births. It is characterized by severe intellectual disability, physical abnormalities, and a high mortality rate. It is not the most common chromosomal anomaly.
**Option B:** Turner syndrome, characterized by the absence of one X chromosome (45,X), affects approximately 1 in 2,500 females. While it is a common chromosomal anomaly, it is not the most common.
**Option C:** Patau syndrome (Trisomy 13) is a rare chromosomal anomaly, occurring in approximately 1 in 10,000 births. It is characterized by severe intellectual disability, physical abnormalities, and a high mortality rate.
**Clinical Pearl / High-Yield Fact**
The risk of chromosomal anomalies increases with advanced maternal age, with the majority of Down syndrome cases occurring in women over the age of 35.
**Correct Answer: D. Trisomy 21, also known as Down syndrome.**