To solve this problem, we need to understand why periodic infusion of genetically engineered lymphocytes is required in the gene therapy of Adenosine Deaminase (ADA) deficiency. Let's analyze the options:$\newline \bullet$Option 1: Retroviral vector is introduced into these lymphocytes.$\newline$ - While retroviral vectors are used to introduce the ADA gene into lymphocytes, this does not explain the need for periodic infusion.$\newline \bullet$Option 2: Gene isolated from marrow cells producing ADA is introduced into cells at embryonic stages.$\newline$ - This option is incorrect because the therapy involves introducing the gene into lymphocytes, not at embryonic stages.$\newline \bullet$Option 3: Lymphocytes from the patient's blood are grown in culture outside the body.$\newline$ - Although lymphocytes are cultured outside the body, this does not address the need for periodic infusion.$\newline \bullet$Option 4: Genetically engineered lymphocytes are not immortal cells.$\newline$ - This is the correct explanation. Lymphocytes have a limited lifespan and do not divide indefinitely. Therefore, periodic infusion is necessary to maintain adequate levels of ADA.$\newline$Thus, the correct option is Option 4: Genetically engineered lymphocytes are not immortal cells.