True regarding Down syndrome is?
**Core Concept**
Down syndrome is a genetic disorder caused by an extra copy of chromosome 21 (trisomy 21), resulting in intellectual disability and various physical abnormalities. This condition is also known as trisomy 21 and is the most common chromosomal disorder.
**Why the Correct Answer is Right**
The extra copy of chromosome 21 leads to overexpression of genes, particularly those involved in brain development and cell growth. This overexpression disrupts normal brain function and development, resulting in intellectual disability and other cognitive impairments. The condition is also associated with increased risk of certain medical conditions, such as heart defects, hearing loss, and leukemia.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to Down syndrome.
**Option B:** This option is not accurate regarding Down syndrome.
**Option C:** This option is incorrect as it does not describe Down syndrome.
**Clinical Pearl / High-Yield Fact**
It's essential to note that Down syndrome is a genetic condition, and the risk increases with maternal age, especially after 35 years. This is because older eggs are more likely to have an extra chromosome.
**Correct Answer: C.**