In the context of recent advances in human reproductive technology, Pronuclear Transfer” is used for

Pronuclear transfer is a step in mitochondrial replacement therapy (MRT). The intended mother's egg and a donor egg are each fertilised in vitro. Soon after fertilisation, each zygote shows two pronuclei, one from the egg and one from the sperm. The nuclear material from the intended mother's zygote is moved into the donor zygote, whose mitochondria are healthy. The embryo then carries the parents' nuclear DNA and the donor's mitochondrial DNA, which protects the child from mitochondrial diseases passed down the maternal line. So D is correct.

• A is wrong. Fertilisation with donor sperm is ordinary IVF or ICSI. It is not the purpose of pronuclear transfer.
• B is wrong. Editing sperm-producing cells is germline modification work, not pronuclear transfer.
• C is wrong. Turning stem cells into embryos belongs to stem cell and embryo model research.

The core concept is maternal inheritance of mitochondrial DNA, which lies outside the nuclear genome. For Mains, it links to bioethics and to the ART (Regulation) Act, 2021. The UK legalised MRT in 2015, and it reported the first births after pronuclear transfer in 2025, after this exam.

Trap: the word "pronuclear" sounds like fertilisation, so aspirants pick A. The point here is replacing defective mitochondria.