Which enzyme deficiency will lead to a disease called SCID?

Options

A. Adenosine deaminase
B. Alcohol dehydrogenase
C. Creatine kinase
D. Myosin ATPase

Answer

### Step-by-Step Solution:

1. **Understanding SCID**:
- SCID stands for Severe Combined Immunodeficiency, which is a genetic disorder characterized by a severely compromised immune system.

2. **Identifying the Enzyme**:
- The disease SCID is specifically caused by the deficiency of an enzyme known as Adenosine Deaminase (ADA).

3. **Function of Adenosine Deaminase**:
- The ADA enzyme plays a crucial role in the metabolism of nucleotides. It converts deoxyadenosine, a toxic compound, into deoxyinosine, which is non-toxic.

4. **Impact of ADA Deficiency**:
- When there is a deficiency of ADA, deoxyadenosine accumulates in the body. This accumulation is toxic to lymphocytes, which are vital cells in the immune system.

5. **Consequences of Lymphocyte Toxicity**:
- The toxicity leads to a reduction in the number of lymphocytes, resulting in impaired immunity. This makes individuals with SCID highly susceptible to infections, even from minor pathogens.

6. **Conclusion**:
- Therefore, the enzyme deficiency that leads to SCID is Adenosine Deaminase (ADA).

### Final Answer:
The enzyme deficiency that leads to SCID is **Adenosine Deaminase (ADA)**.

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