Which condition is most likely to be diagnosed based on sharply defined plaques and scaly skin?

The diagnosis of psoriasis is supported by the presence of sharply defined plaques and scaly skin. This autoimmune condition is characterized by well-defined, raised areas of red skin covered with silvery-white scales, often appearing on the elbows, knees, and scalp. The sharp differentiation of these plaques is a hallmark of psoriasis, making it stand out from other skin conditions.

Atopic dermatitis typically presents with more diffuse, less sharply defined lesions and often involves intense itching, particularly in children, and is often associated with a history of allergy or asthma.

Contact dermatitis arises due to direct skin exposure to an irritant or allergen, resulting in inflammation that may present as red, itchy, or burning skin but does not usually exhibit the thick plaques characteristic of psoriasis.

Eczema can refer broadly to a range of conditions but is commonly associated with the itching, dryness, and inflammation of the skin rather than the clearly demarcated plaques seen in psoriasis.

Thus, the distinctive features of psoriasis, particularly the sharply defined plaques combined with scaling, are what substantiate its diagnosis in this scenario.