What is characterized as an open lesion on the skin that may require medical referral?

An ulcer is characterized as an open lesion on the skin, which presents as a sore or wound that has broken down tissue. It typically extends into the deeper layers of the skin and can be a sign of underlying health issues or infections, hence the likelihood that it may require medical referral for proper diagnosis and treatment. Because ulcers can be caused by various factors, including trauma, disease, or infection, medical intervention is often necessary to address the underlying causes and to promote healing.

In contrast, crust refers to a dried substance on the skin, usually composed of serum, blood, or pus, which does not imply an opening in the skin. A vesicle is a small, fluid-filled sac that is typically less severe and often heals without medical intervention. A fissure is a small tear or crack in the skin, often found in areas that experience dryness or friction but is not necessarily classified as an open lesion needing immediate referral.