Read the following text and answer the following questions on the basis of the same: Down syndrome (sometimes called Down's syndrome) is a condition in which a child is born with an extra copy of their 21st chromosome hence its other name, trisomy 21. The affected individual mental retarded, short statured with small round, head, furrowed tongue and partially open mouth, Physical, psychomotor and mental development is retarded.
Down syndrome is caused due to

Options

A. bacterial infection
B. a chromosomal abnormality lack of oxygen supply to the brain during birth
C. Viral infection
D. a chromosomal abnormality

Explanation

**Step-by-Step Solution:**

1. **Understanding Down Syndrome:**
- Down syndrome is a genetic condition characterized by the presence of an extra copy of the 21st chromosome, which is referred to as trisomy 21.

2. **Identifying Symptoms:**
- Individuals with Down syndrome often exhibit specific physical traits such as short stature, a small round head, a furrowed tongue, and a partially open mouth. They also experience delays in physical, psychomotor, and mental development.

3. **Analyzing the Cause:**
- The cause of Down syndrome is linked to a chromosomal abnormality. Specifically, it occurs due to a phenomenon known as nondisjunction, where the homologous chromosomes fail to separate properly during cell division (meiosis). This results in gametes with an abnormal number of chromosomes.

4. **Connecting to the Question:**
- The question asks for the cause of Down syndrome from the given options:
A. Bacterial infection
B. Lack of supply of oxygen to the brain during birth
C. A viral infection
D. A chromosomal abnormality

5. **Choosing the Correct Answer:**
- Based on the understanding that Down syndrome is caused by a chromosomal abnormality (specifically trisomy 21 due to nondisjunction), the correct answer is **D. A chromosomal abnormality**.