## Core Concept
Graft-versus-host disease (GVHD) is a major complication of allogeneic hematopoietic stem cell transplantation. It occurs when immune cells from the transplanted graft recognize the recipient's body as foreign and mount an immune response against it. This reaction involves the activation of T lymphocytes from the graft.

## Why the Correct Answer is Right
The correct answer, although not directly provided, needs to be evaluated based on common statements about GVHD. Typically, GVHD is characterized by its occurrence after allogeneic transplantation, involvement of skin, liver, and gastrointestinal tract, and the role of T cells in its pathogenesis. A statement that is not true about GVHD would involve incorrect information regarding its pathophysiology, risk factors, clinical manifestations, or prevention strategies.

## Why Each Wrong Option is Incorrect
Without specific details on options A, B, C, and D, a general approach to evaluating incorrect statements about GVHD is provided:
- **Option A:** If a statement claims GVHD only occurs immediately after transplantation, it would be incorrect because GVHD can occur early (acute) or late (chronic) after transplantation.
- **Option B:** If a statement suggests GVHD is not a significant concern in syngeneic transplants, it would be correct, not incorrect, as GVHD is less common in syngeneic (identical twin) transplants due to the lack of genetic disparity.
- **Option C:** If a statement says T cells do not play a role in GVHD, it would be incorrect because T cells are central to the development of GVHD.
- **Option D:** If a statement claims GVHD only affects the skin, it would be incorrect because GVHD can affect multiple organs, including the skin, liver, and gastrointestinal tract.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **GVHD prophylaxis** often involves immunosuppressive drugs and T-cell depletion strategies. Clinicians must balance the risk of GVHD with the risk of graft failure and relapse of the underlying disease.

## Correct Answer Line
**Correct Answer: D.**