**Core Concept:**
Graft-versus-host disease (GVHD) is a complication that occurs after an allogeneic stem cell transplantation when the transplanted immune cells attack the recipient's tissues and organs. Biopsy is a way to diagnose GVHD by examining tissue samples under a microscope.

**Why the Correct Answer is Right:**
The correct answer is based on the organs primarily affected by GVHD. GVHD typically involves the skin, liver, and gastrointestinal tract, while kidney involvement is less common. Therefore, a kidney biopsy cannot be used to prove GVHD.

**Why Each Wrong Option is Incorrect:**
A. Liver: Although GVHD can affect the liver, a liver biopsy is commonly used to diagnose GVHD, making it a more appropriate choice for proving GVHD compared to the other options.
B. Skin: Biopsy of the skin is often performed to diagnose GVHD, especially in cases with skin involvement, which is a common manifestation of GVHD.
C. Kidney: Kidney biopsy is less common in diagnosing GVHD, as kidney involvement is less frequent compared to skin, liver, and gastrointestinal tract involvement.
D. Gastrointestinal tract: Gastrointestinal involvement is one of the primary sites of GVHD manifestation, making a biopsy of the gastrointestinal tract a relevant tool for proving GVHD.

**Clinical Pearl:**
In cases of suspected GVHD, a comprehensive evaluation involving biopsies of skin, liver, and gastrointestinal tract is essential for accurate diagnosis and management. While kidney biopsy might be considered in specific cases, it is less commonly used compared to biopsies from these three organs.