Each of the following lesions may result from chronic mucosal IRRITATION and belongs in that differential diagnosis EXCEPT:

Correct answer: A. Lichen planus

• A. CORRECT as the EXCEPTION. LICHEN PLANUS is a chronic T-CELL MEDIATED AUTOIMMUNE condition, not a reactive response to trauma. Cytotoxic T lymphocytes attack basal keratinocytes, producing a band-like subepithelial infiltrate and Civatte bodies histologically, with bilateral Wickham's striae clinically. Its cause is immunological, not mechanical. (Note the important distinction from lichenoid reactions, which ARE triggered by amalgam contact or drugs.)
• B. Irritation fibroma is the archetypal reactive lesion — a fibrous hyperplastic nodule arising at a site of repeated trauma, typically the buccal mucosa along the occlusal line.
• C. Morsicatio buccarum is the direct product of a chronic parafunctional habit — the shredded, macerated white line along the occlusal plane is mechanical trauma by definition.
• D. Leukoedema is a variation of normal appearing as a diffuse grey-white filmy opacity that DISAPPEARS ON STRETCHING. It is more pronounced in smokers and with local irritation.

Note: Key contrast: reactive/traumatic lesions are UNILATERAL and located exactly where the irritant sits. Lichen planus is BILATERAL and roughly symmetrical — that symmetry is your best clinical clue that the cause is immune, not mechanical.