Final answer:
A stage I pressure ulcer is characterized by nonblanching erythema and intact skin; it is typically reversible with proper care. Tissue hardening and swelling are not defining characteristics of this stage.
Step-by-step explanation:
Characteristics of a stage I pressure ulcer include nonblanching erythema, which means redness that does not lighten or disappear when pressure is applied to the area, indicating that a pressure ulcer may be starting to form. This stage is also known for the skin being intact with no open wounds, and the condition is often reversible with proper care and pressure relief. Swelling may occur, but it is not a defining characteristic, and while the area may feel different (warmer or cooler) compared to adjacent tissue, tissue hardening is not typically associated with stage I pressure ulcers.