Which of the following is a key feature of pitting edema?

Pitting edema is characterized by the presence of an observable indentation when pressure is applied to the swollen area of the body. This occurs because the excess fluid in the interstitial spaces can be displaced, leading to a temporary indentation that remains even after the pressing force is removed. This feature is distinct from other types of edema, where the swelling may be more generalized or firm, and no indentation is seen.

Bright red discoloration typically indicates inflammation or irritation rather than edema itself, while localized warmth might suggest infection or a deep vein thrombosis. Hardness of tissue is indicative of non-pitting edema or chronic conditions where swelling is due to other factors like fibrotic changes rather than fluid accumulation. Thus, the defining characteristic of pitting edema is indeed the observable indentation, confirming the accuracy of the chosen answer.