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

Reason (R) : Lymphocytes are immortal.

Correct Option: The correct answer is "Assertion (A) is true, but Reason (R) is false."

Step-by-Step Explanation:

1. Analysis of Assertion (A):
Adenosine Deaminase (ADA) deficiency is a genetic disorder caused due to the deletion of the gene coding for the enzyme adenosine deaminase, which is crucial for the proper functioning of the immune system.
One of the clinical approaches for treating ADA deficiency is gene therapy. In this process, lymphocytes (a type of white blood cell) are isolated from the patient's blood and grown in a culture outside the body. A functional ADA complementary DNA (cDNA) is then introduced into these cultured lymphocytes using a viral vector.
Finally, these genetically engineered lymphocytes are returned to the patient. However, because lymphocytes are not immortal and have a finite life span, the patient requires periodic infusion of these genetically engineered lymphocytes. Thus, Assertion (A) is true.

2. Analysis of Reason (R):
Lymphocytes are eukaryotic cells that perform immune functions and have a limited, finite lifespan; they are mortal. Because they naturally die over time, the genetically engineered lymphocytes injected into the patient eventually deplete, which is the very reason why the therapy requires periodic repetitions. Thus, Reason (R) is false.

Conclusion:
Since Assertion (A) is a scientifically correct statement and Reason (R) is a scientifically incorrect statement, the correct option is "Assertion (A) is true, but Reason (R) is false."