Which skin condition is often associated with a well-demarcated, scaly plaque?

The skin condition associated with a well-demarcated, scaly plaque is psoriasis vulgaris. In this condition, patients typically present with raised, red plaques covered with silvery-white scales, most commonly seen on the elbows, knees, and scalp. These plaques are well-defined and can vary in size, making the demarcation between the healthy skin and the affected areas very clear.

Psoriasis vulgaris is a chronic inflammatory disease characterized by an overproduction of skin cells, leading to the buildup of these plaques. The well-demarcated nature of the lesions is a key diagnostic feature, distinguishing it from other skin conditions that may have less distinct borders or different scaling characteristics.

Bowen's disease, pityriasis rosea, and discoid eczema exhibit other physical characteristics or patterns that do not align with the classic presentation of psoriasis vulgaris. For instance, Bowen’s disease typically presents as a solitary, crusted lesion, while pityriasis rosea often starts with a herald patch followed by a more diffuse rash. Discoid eczema may present with coin-shaped lesions but lacks the distinct scales and inflammatory nature seen in psoriasis. Thus, psoriasis vulgaris is the most fitting choice for the description provided.