**Question:** A woman receives a kidney transplant from her sister who is an identical twin. What type of graft is it?

**Core Concept:** Kidney transplantation, HLA compatibility, identical twins.

**Why the Correct Answer is Right:** In this scenario, the woman receives a kidney transplant from her sister who is an identical twin. Identical twins have identical HLA (Human Leukocyte Antigen) types due to their shared genetic makeup. HLA compatibility plays a crucial role in successful organ transplantation, as it ensures minimal immune system rejection of the graft. Since the donor and recipient share identical HLA types, the transplant will have a higher probability of success compared to mismatched transplants.

**Why Each Wrong Option is Incorrect:**

A. **Not Mismatched (Option A):** This option is incorrect because the donor and recipient share identical HLA types, reducing the chances of immune system rejection.
B. **Not from a related donor (Option B):** This option is incorrect because the transplant comes from a related donor (sister), making it more likely to be successful due to HLA matching.
C. **Not from a deceased donor (Option C):** This option is incorrect because the type of graft remains the same – a living donor transplant – regardless of whether the donor is deceased or not.
D. **Not from a deceased donor (Option D):** This option is a repetition of Option C and does not add relevant information about HLA matching.

**Clinical Pearl:** In this scenario, the type of graft is significant due to the HLA compatibility between the donor and recipient. Successful organ transplantation depends on factors like HLA matching, donor-recipient relationship, and graft source (deceased or living). For a living donor, HLA matching significantly increases the chances of transplantation success, especially when the donor and recipient are genetically identical. This concept is crucial in organ transplantation, ensuring the patient does not reject the graft due to immune system recognition of foreign antigens.