Mangolian idiocy are due to trisomy in `21^(st)` chromosome is called:

Options

A. Down's syndrome
B. Turner's syndrome
C. Kleinfelters syndrome
D. Triplex syndrome

Answer

To solve the question regarding the condition known as "Mongolian idiocy," which is due to trisomy of the 21st chromosome, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Trisomy**:
- Trisomy refers to the presence of an extra chromosome in a pair. In this case, we are focusing on the 21st chromosome.

2. **Identifying the Condition**:
- The condition associated with trisomy of the 21st chromosome is characterized by specific physical and intellectual disabilities. This condition is commonly referred to in medical literature.

3. **Recognizing the Name**:
- The term "Mongolian idiocy" is an outdated term that was historically used to describe Down syndrome. The modern and accepted term for this condition is Down syndrome.

4. **Reviewing the Options**:
- The options given are:
A. Down syndrome
B. Turner syndrome
C. Klinefelter syndrome
D. Triploidy syndrome
- We need to identify which of these options corresponds to the trisomy of the 21st chromosome.

5. **Eliminating Incorrect Answers**:
- Turner syndrome (Option B) is a condition where females have only one X chromosome (45, X).
- Klinefelter syndrome (Option C) occurs in males with an extra X chromosome (47, XXY).
- Triploidy syndrome (Option D) involves having three sets of chromosomes (69 chromosomes total) and is not related to the 21st chromosome.

6. **Selecting the Correct Answer**:
- Based on the understanding that trisomy of the 21st chromosome is known as Down syndrome, we can conclude that the correct answer is:
**A. Down syndrome**

### Final Answer:
**A. Down syndrome**