Which sign is indicative of psoriasis?

The Auspitz sign is specifically indicative of psoriasis and is characterized by the presence of pinpoint bleeding when psoriatic plaques are scraped off. This occurs because the epidermis is thickened and inflamed in psoriasis, leading to the disruption of the small blood vessels just beneath the skin surface. When the lesions are manipulated or removed, these blood vessels are exposed, resulting in bleeding.

The Koebner phenomenon, while associated with psoriasis, is broader in that it represents the development of psoriatic lesions at sites of trauma or injury on the skin, rather than being a specific sign like the Auspitz sign.

The Nickel Test is used to evaluate allergic contact dermatitis and does not have a direct correlation with psoriasis, focusing instead on allergic reactions to substances like nickel.

Wickham striae are fine white lines seen in lichen planus and are not associated with psoriasis, making them irrelevant in the context of identifying this condition.