## **Core Concept**
The question revolves around the historical milestone of gene therapy, specifically focusing on the first successful application of somatic enzyme gene therapy. Gene therapy involves making targeted changes to a person's genome to treat or cure diseases. Somatic enzyme gene therapy targets non-reproductive cells to correct a genetic deficiency.

## **Why the Correct Answer is Right**
The first successful gene therapy was performed to treat **Severe Combined Immunodeficiency (SCID)**, specifically the adenosine deaminase (ADA) deficiency form. This condition leads to a severe impairment of the immune system. The therapy aimed to introduce a functional ADA gene into the patient's somatic cells, thereby enabling the production of the ADA enzyme and improving immune function. This pioneering work was conducted by **William French Anderson** and his team in 1990.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Not specified due to lack of information.
- **Option B:** Not specified due to lack of information.
- **Option D:** Not specified due to lack of information.

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that the first gene therapy was for **ADA deficiency**, a form of SCID. This historic treatment marked the beginning of gene therapy as a potential treatment for genetic diseases. The patient, **Ashanti DeSilva**, received modified T cells with the ADA gene, showing the feasibility of gene therapy in correcting genetic defects.

## **Correct Answer:** C. Severe Combined Immunodeficiency (SCID) due to ADA deficiency.