Which of the following features is found in hairy tongue?

In the context of hairy tongue, the defining feature is the presence of elongated filiform papillae. This occurs when the filiform papillae, which normally have a short and smooth appearance, become excessively long due to factors such as poor oral hygiene, smoking, or certain medications. This elongation leads to a characteristic hair-like appearance on the tongue's surface, which is why the condition is referred to as "hairy tongue."

Each of the other features mentioned can occur in different oral conditions but is not specific to hairy tongue. Patchy atrophy of filiform papillae is more indicative of conditions like geographic tongue or can occur due to nutritional deficiencies. A red inflamed tongue usually suggests inflammation or irritation, which is not a sign of hairy tongue itself. Lastly, a white coating can occur in various other conditions, such as oral thrush or leukoplakia, but is not a typical feature associated with hairy tongue. Therefore, the elongation of filiform papillae is the key characteristic that confirms the diagnosis of hairy tongue.