What is the primary characteristic of psoriasis lesions?

The primary characteristic of psoriasis lesions is the presence of reddish papules that form plaques. Psoriasis is a chronic skin condition that leads to the rapid renewal of skin cells, resulting in raised, inflamed patches on the skin's surface. These plaques are typically covered with silvery-white scales and often appear on areas such as the elbows, knees, and scalp.

This choice accurately reflects the typical presentation of psoriasis lesions, with their characteristic reddish color and raised, thickened skin structure. The formation of plaques distinguishes psoriasis from other skin conditions that might involve different types of lesions or formations.

Other options describe characteristics of different skin conditions. Flat red spots might suggest a rash or other dermatoses, warts with central depressions are more indicative of viral papillomas, and blisters filled with clear fluid are representative of conditions like herpes or contact dermatitis. Therefore, the description of reddish papules forming plaques is the hallmark of psoriasis.