Final answer:
A crust on the skin typically results from the body's healing process after injury, inflammation, or infection, with dried serum, blood, or pus-forming the crust itself.
Step-by-step explanation:
The cause of a crust forming on the skin is part of the body's natural healing process following an injury, inflammation, or infection. When the skin is damaged, pathogens can enter and cause infection, which can further damage tissues, including hair follicles and glands. The defensive inflammatory response results in inflammation and sometimes a crust, which is the dried remnants of serum, blood, or pus. Conditions like folliculitis and acne involve inflammation of hair follicles, which can lead to the formation of pustules and eventually crusts. Similarly, in conditions like Crohn's disease-associated BADAS, pustules, and crusts can form around areas of previous inflammation or injury.