**Core Concept**
The Koebner phenomenon refers to the appearance of skin lesions on areas of the body that have been traumatized, such as cuts, burns, or scrapes, in individuals with certain skin conditions, including psoriasis and vitiligo. This phenomenon is a result of the body's immune response to trauma and the subsequent activation of immune cells.

**Why the Correct Answer is Right**
The Koebner phenomenon is characterized by the development of new skin lesions at the site of trauma in individuals with autoimmune skin conditions. This occurs due to the activation of immune cells, such as T cells and dendritic cells, which recognize the skin trauma as a foreign insult and trigger an immune response. The resulting inflammation and immune cell infiltration lead to the development of new lesions. In the case of psoriasis, the Koebner phenomenon is thought to be mediated by the activation of T cells and the release of cytokines, such as tumor necrosis factor-alpha (TNF-alpha), which promote inflammation and skin cell proliferation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the Koebner phenomenon is not a characteristic of seborrheic dermatitis, a skin condition characterized by inflammation and flaking of the skin, particularly on the scalp, face, and torso.
**Option B:** This option is incorrect as the Koebner phenomenon is not a characteristic of contact dermatitis, a skin condition caused by an allergic reaction to a substance that comes into contact with the skin.
**Option C:** This option is incorrect as the Koebner phenomenon is not a characteristic of acne, a skin condition caused by the blockage of oil glands in the skin.

**Clinical Pearl / High-Yield Fact**
The Koebner phenomenon is a classic example of the body's immune response to trauma and can be a useful diagnostic clue in patients with autoimmune skin conditions.

**Correct Answer: D. Psoriasis**