SCID is caused by defective gene coding for enzyme

Options

A. Adenosine deaminase
B. Adensoine transaminase
C. Adenosine transferase
D. Guanosine transaminase

Answer

### Step-by-Step Solution:

1. **Understanding SCID**: SCID stands for Severe Combined Immunodeficiency. It is a genetic disorder that leads to a significant impairment of the immune system, specifically affecting the development of B and T lymphocytes.

2. **Identifying the Cause**: The disorder is caused by mutations in genes that are crucial for the immune system's functioning. One of the key genes involved is the one that codes for the enzyme adenosine deaminase (ADA).

3. **Role of Adenosine Deaminase**: Adenosine deaminase is an enzyme that is essential for the breakdown of purines, which are vital components of nucleotides. When the ADA gene is defective, it leads to a lack of this enzyme.

4. **Consequences of ADA Deficiency**: The absence of functional ADA results in the accumulation of toxic metabolites, specifically deoxyadenosine triphosphate (dATP). High levels of dATP inhibit the enzyme ribonucleotide reductase, which is necessary for the conversion of ribonucleotides to deoxyribonucleotides.

5. **Impact on Lymphocytes**: The inhibition of ribonucleotide reductase affects lymphocyte proliferation, leading to a reduction in the number of functional B and T cells. This results in the immunodeficiency characteristic of SCID.

6. **Conclusion**: Based on the information, the defective gene responsible for SCID codes for the enzyme adenosine deaminase. Thus, the correct answer to the question is adenosine deaminase.

### Final Answer:
The defective gene coding for the enzyme causing SCID is **adenosine deaminase**.

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