How is an ulcer defined in dermatological terms?

An ulcer in dermatological terms is correctly defined as an open lesion that involves the loss of epidermis and can extend into the dermis or deeper layers of the skin. The presence of pus indicates that there may be a bacterial infection or inflammatory process, which is often associated with ulcers. This characteristic of being an open wound where the skin's surface is disrupted, along with the potential for purulent discharge, distinguishes it from other skin lesions.

The other descriptions do not accurately capture the essence of an ulcer. For instance, a raised area on the skin may pertain to conditions like papules or plaques, which do not have the open, eroded surface that characterizes ulcers. A painless bump under the skin could refer to a cyst or lipoma, neither of which involve ulceration. Lastly, a small red spot caused by broken capillaries aligns more with petechiae or purpura, which are not open lesions and do not involve the loss of skin tissue. Therefore, the correct answer reflects a comprehensive understanding of the nature and presentation of skin ulcers.