Which of the following conditions is known to be precancerous?

Leukoplakia is recognized as a precancerous condition because it involves the formation of white patches in the oral cavity, which can result from irritation, inflammation, or tobacco use. The significance of leukoplakia lies in its potential to undergo malignant transformation, which means that if left unchecked, it can lead to oral squamous cell carcinoma. The presence of leukoplakia warrants careful examination and monitoring by dental professionals to determine the nature of the lesions and to assess any risk of cancer development.

In contrast, the other conditions listed—oral candidosis, angular cheilitis, and glossitis—do not carry the same level of risk for becoming cancerous. Oral candidosis is a fungal infection, angular cheilitis is typically associated with skin irritation or deficiencies (like vitamin B2 or iron), and glossitis refers to inflammation of the tongue which may result from various causes such as allergies, infections, or systemic diseases. While these conditions can indicate underlying health issues, they are not classified as precancerous. Thus, recognizing leukoplakia's association with cancer risk is important for oral health professionals in identifying patients who may need further evaluation and intervention.