Mongolism syndrome is caused by

Options

A. One extra chromosome
B. One extra sex chromosome
C. One extra chromosome in 21st pair
D. One less sex chromosome

Explanation

**Step-by-Step Solution:**

1. **Understanding Mongolism Syndrome**:
- Mongolism syndrome is more commonly known as Down syndrome. It is a genetic disorder that affects physical and intellectual development.

2. **Identifying the Cause**:
- Down syndrome is primarily caused by an abnormality in chromosome number. Specifically, it is associated with the presence of an extra chromosome.

3. **Chromosomal Abnormality**:
- In the case of Down syndrome, there is a specific chromosomal abnormality known as trisomy 21. This means that instead of the usual two copies of chromosome 21, individuals with Down syndrome have three copies.

4. **Conclusion**:
- Therefore, Mongolism syndrome (Down syndrome) is caused by the presence of an extra chromosome, specifically an extra copy of chromosome 21.

5. **Final Answer**:
- Mongolism syndrome is caused by trisomy 21, which is the presence of an extra chromosome 21.

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