ADA deficiency can permanently be cured by:

The correct answer is Option (4) → Gene from marrow cells producing ADA is introduced into cells at embryonic stage

ADA deficiency is a disorder of the immune system caused by the deletion of the gene for the enzyme adenosine deaminase (ADA). Gene therapy treats a genetic disorder by putting a normal gene into the cells of the patient.

(1) Periodic infusion of genetically engineered lymphocytes — Incorrect. Lymphocytes of the patient are given a working ADA gene and put back into the body. But lymphocytes do not live for ever, so this has to be repeated from time to time. It is not a permanent cure.

(2) Enzyme replacement therapy — Incorrect. Working ADA enzyme is given to the patient by injection. It treats the disease but does not cure it completely.

(3) Bone marrow transplantation — Incorrect. It can cure some children, but NCERT groups it with enzyme replacement as not completely curative.

(4) Gene from marrow cells producing ADA is introduced into cells at embryonic stage — Correct. If the gene is put into cells at an early embryonic stage, all the cells that develop from them carry it. This could be a permanent cure.

NCERT Extract:
Gene Therapy – “… The first clinical gene therapy was given in 1990 to a 4-year old girl with adenosine deaminase (ADA) deficiency. This enzyme is crucial for the immune system to function. The disorder is caused due to the deletion of the gene for adenosine deaminase. In some children ADA deficiency can be cured by bone marrow transplantation; in others it can be treated by enzyme replacement therapy, in which functional ADA is given to the patient by injection. But the problem with both of these approaches that they are not completely curative. As a first step towards gene therapy, lymphocytes from the blood of the patient are grown in a culture outside the body. A functional ADA cDNA (using a retroviral vector) is then introduced into these lymphocytes, which are subsequently returned to the patient. However, as these cells are not immortal, the patient requires periodic infusion of such genetically engineered lymphocytes. However, if the gene isolate from marrow cells producing ADA is introduced into cells at early embryonic stages, it could be a permanent cure.”