**Core Concept**
Koebner's phenomenon refers to the development of skin lesions on areas of the body that have been traumatized, such as cuts, scratches, or burns, in patients with certain skin conditions. This phenomenon is a result of the body's abnormal response to injury in individuals with predisposed skin conditions.

**Why the Correct Answer is Right**
Koebner's phenomenon is a characteristic feature of psoriasis, vitiligo, and lichen planus, among other conditions. In these diseases, the skin's response to injury triggers the formation of new lesions, which can manifest as psoriatic plaques, vitiligo patches, or lichen planus bands. This phenomenon highlights the abnormal immune response and skin reactivity in these conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while psoriasis is a condition that exhibits Koebner's phenomenon, it is not the only condition listed. The question asks for the exception, so options that are known to exhibit Koebner's phenomenon cannot be the correct answer.
**Option B:** This option is incorrect because vitiligo is a condition that is well-known to exhibit Koebner's phenomenon, making it an unlikely correct answer.

**Clinical Pearl / High-Yield Fact**
Koebner's phenomenon is a valuable clinical clue that can help diagnose skin conditions, particularly in cases where the primary lesion is not evident. It is essential to consider this phenomenon when evaluating patients with new skin lesions, especially in the context of trauma or injury.

**Correct Answer: A.**