Which transplant type involves a donor who is an identical twin?

The correct answer is a syngeneic transplant. In this type of transplant, the donor is a genetically identical twin of the recipient. This means that the genetic makeup of the donor and recipient is the same, allowing for the most compatible match possible. This is significant because it minimizes the risk of graft-versus-host disease (GVHD) and rejection, as the immune system is less likely to identify the transplanted cells as foreign.

In the context of bone marrow or stem cell transplants, syngeneic transplants are specifically advantageous for patients who need blood or marrow from a donor, as the identical genetic match leads to better outcomes in terms of acceptance and function of the transplanted material.

Other transplant types do not utilize a genetically identical donor. An allogenic transplant involves a donor who is not related or genetically similar to the recipient, which can increase the risk of complications such as GVHD. An autologous transplant refers to a scenario where the recipient's own cells are collected and reinfused, which is not applicable in the case of a twin donor. Hence, the choice of syngeneic transplant as the answer reflects the unique relationship between the donor and recipient that defines this type of transplant.