A white lesion is present on the attached gingiva in the region of the left maxillary premolars. Each of the following belongs in the differential diagnosis EXCEPT one. Which is the EXCEPTION?

Correct answer: C. Actinic keratosis

• A. This BELONGS. Leukoedema is a common greyish-white filmy change that classically DISAPPEARS ON STRETCHING the mucosa. It is most typical of the buccal mucosa but is a legitimate differential for a white mucosal change.
• B. This BELONGS. Leukoplakia is by definition a white patch that cannot be scraped off and cannot be attributed to any other condition — a diagnosis of exclusion, and gingiva is a recognised site.
• C. CORRECT — this is the EXCEPTION. ACTINIC KERATOSIS (actinic cheilitis when it affects the lip) is caused by chronic ULTRAVIOLET exposure. It therefore occurs only on sun-exposed surfaces — the skin and the vermilion of the lower lip. It cannot arise on the attached gingiva, which receives no sunlight.
• D. This BELONGS. Pseudomembranous candidiasis produces white plaques that WIPE OFF leaving an erythematous base — the wipe test separates it immediately from leukoplakia.

Note: A workable sorting algorithm for an oral white lesion: does it wipe off (candidiasis, slough, burn)? Does it disappear on stretching (leukoedema)? Is it bilateral with Wickham striae (lichen planus)? Is there frictional or chemical cause (frictional keratosis, morsicatio, tobacco pouch)? If none of these apply and it persists beyond two weeks — biopsy.