**Core Concept**
The Graft-Versus-Host Disease (GVHD) is a major complication of allogenic hematopoietic stem cell transplantation, where the immune cells in the graft (donor) recognize the recipient's tissues as foreign, leading to an immune response. GVHD can affect various organs, including the skin, gastrointestinal tract, liver, and lungs.

**Why the Correct Answer is Right**
The lungs are particularly susceptible to GVHD due to the high density of immune cells, including T cells, which play a crucial role in the pathogenesis of GVHD. In GVHD, the donor T cells recognize the recipient's major histocompatibility complex (MHC) molecules as foreign, leading to activation and proliferation of these T cells. The activated T cells then release cytokines, such as interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α), which cause inflammation and damage to the lung tissue. This results in symptoms such as dyspnea, cough, and pulmonary infiltrates.

**Why Each Wrong Option is Incorrect**
**Option A:** While the skin is a common site of GVHD, it is not the most affected organ. The skin manifestations of GVHD typically occur early in the disease process, whereas the lungs are more frequently affected in the later stages.

**Option B:** The gastrointestinal tract is a frequent site of GVHD, particularly in the acute phase of the disease. However, the lungs are more commonly affected in the chronic phase of GVHD.

**Option C:** The liver is also a common site of GVHD, and liver dysfunction can occur in up to 50% of patients with GVHD. However, the lungs are more frequently affected in terms of morbidity and mortality.

**Clinical Pearl / High-Yield Fact**
The severity and frequency of GVHD can be reduced by using immunosuppressive medications, such as corticosteroids and calcineurin inhibitors, as part of the conditioning regimen before transplantation.

**✓ Correct Answer: D. >Lung**