**Core Concept**
Graft-versus-host disease (GVHD) is a major complication of allogenic hematopoietic stem cell transplantation, where donor lymphocytes recognize the recipient's tissues as foreign and mount an immune response. GVHD can occur either acutely or chronically, with the former typically occurring within the first 100 days post-transplant.

**Why the Correct Answer is Right**
GVHD is caused by the immune recognition of MHC (Major Histocompatibility Complex) molecules on the recipient's cells by donor T cells. The recipient's MHC molecules are recognized as foreign by the donor T cells, leading to an immune response against the recipient's tissues. This process is mediated by CD4+ and CD8+ T cells, which recognize and target the recipient's cells for destruction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because GVHD is not caused by the recipient's immune system recognizing the donor's cells as foreign. Instead, it is the donor's immune cells that recognize the recipient's cells as foreign.
**Option B:** This option is incorrect because GVHD is not primarily caused by the recipient's immune system rejecting the graft. While immune rejection can occur, GVHD is a distinct entity that involves the donor's immune cells attacking the recipient's tissues.
**Option C:** This option is incorrect because GVHD is not solely caused by the use of immunosuppressive drugs. While immunosuppression can increase the risk of GVHD, it is not the primary cause of the disease.

**Clinical Pearl / High-Yield Fact**
A key aspect of GVHD is the recognition of MHC molecules on the recipient's cells by donor T cells, which highlights the importance of HLA matching between donor and recipient in reducing the risk of GVHD.

**Correct Answer:** D.