What is the major problem if a person lacks ADA?

Explanation

### Step-by-Step Solution

1. **Identify the Enzyme**: The enzyme in question is Adenosine Deaminase (ADA).

2. **Understand the Function of ADA**: ADA is responsible for the conversion of deoxyadenosine, a toxic substance, into deoxyinosine, which is less harmful.

3. **Recognize the Toxicity of Deoxyadenosine**: Deoxyadenosine is toxic to lymphocytes, which are crucial white blood cells involved in the immune response.

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