What is purpura characterized by?

Purpura is specifically characterized by the presence of purple spots on the skin or mucous membranes that do not blanch or turn white when pressed. These spots result from bleeding under the skin, often due to conditions that affect blood vessels and clotting mechanisms. This key feature distinguishes purpura from other skin lesions; the inability to blanch indicates that the discoloration is due to blood pooling in the tissues rather than superficial conditions.

In contrast, fluid-filled blisters involve a different pathology, typically associated with conditions such as dermatitis. Round flat dark spots may refer to other types of skin lesions, like nevi or liver spots, not related to bleeding. Bright red patches in the conjunctiva are indicative of conditions such as conjunctival hemorrhage, rather than purpura, which is strictly associated with the purple hue and the non-blanching characteristic of the spots. Thus, the definition of purpura is clear and distinct, making it the appropriate choice in the context of this question.