Identify a common sign of gingivitis.

A common sign of gingivitis is the presence of red, swollen gums that may bleed upon brushing or flossing. This condition reflects inflammation of the gum tissues due to the buildup of plaque, which harbors bacteria that irritate the gums. Gingivitis is an early form of periodontal disease and is characterized by the body's immune response to this bacterial accumulation.

When gums are healthy, they appear firm and pale pink; however, in gingivitis, the inflammation causes the gums to become more vascularized, resulting in redness and swelling. The bleeding, especially when engaging in dental hygiene practices such as brushing and flossing, is an important clinical indicator that signifies the need for improved oral care and possibly professional dental intervention.

The other signs mentioned, like yellowed teeth or severe tooth sensitivity, do not specifically indicate gingivitis. A sticky film on teeth is associated with plaque accumulation but does not directly reference the inflammatory condition of the gums.