Assertion (A) : A patient of ADA deficiency undergoing treatment for gene therapy requires periodic infusion of genetically engineered lymphocytes. Reason (R) : Lymphocytes are immortal.

The correct option is: Assertion (A) is true, but Reason (R) is false.

Let us break down the scientific concepts behind both the Assertion and the Reason to understand why this option is correct.

1. Analysis of Assertion (A):
Adenosine Deaminase (ADA) deficiency is a genetic disorder caused by the deletion of the gene coding for the enzyme adenosine deaminase, which is crucial for the functioning of the immune system. One approach to treating this disorder is gene therapy. In this process, lymphocytes (a type of white blood cell) are extracted from the patient's blood and grown in a culture outside the body. A functional ADA complementary DNA (cDNA) is then introduced into these lymphocytes using a viral vector, and the genetically engineered cells are infused back into the patient. However, because lymphocytes are not immortal, they eventually die off. Consequently, the patient requires periodic infusions of these genetically engineered lymphocytes to maintain a functioning immune system. Therefore, Assertion (A) is true.

2. Analysis of Reason (R):
Lymphocytes, like most somatic cells in the human body, have a finite lifespan and are mortal cells. They are not immortal. Because they die and must be continuously replaced, the therapeutic effects of infusing genetically engineered lymphocytes are temporary, necessitating periodic treatments. Therefore, Reason (R) is false.

Conclusion:
Since the assertion statement is scientifically accurate and the reason statement is factually incorrect, the correct choice is that Assertion (A) is true, but Reason (R) is false.