Which stain appears as a thin black line along the gingival margin?

The stain that appears as a thin black line along the gingival margin is known as black stain. This type of stain is commonly associated with the presence of iron and can be found in individuals with good oral hygiene, usually forming a distinct line that outlines the gum line. It's important to note that black stain is not harmful to the teeth or gums, but rather is often a cosmetic issue that can be easily removed through professional dental cleaning.

Understanding the context of the other options can clarify why black stain is the correct choice. Green stain typically appears in various shades of green on teeth, often due to the presence of chromogenic bacteria or poor oral hygiene, rather than forming a defined line at the gingival margin. Brown stain, on the other hand, can result from various dietary sources or habits, like smoking, but it generally does not present as a thin line along the margins. Red stain is rare and not typically associated with any common oral hygiene conditions, making it an unlikely contender for this particular description.

Recognizing the characteristics of different types of dental stains helps in both identifying and addressing them during dental practice.