A patient on phenytoin has gingival overgrowth covering all but the incisal third of the teeth. All of the following are plausible contributing causes EXCEPT:

Correct answer: C. Primary herpetic gingivostomatitis

• A. IS a cause. Mouth breathing produces chronic dehydration and irritation of the anterior gingiva, causing localised inflammatory enlargement that compounds the drug effect.
• B. IS a cause. Plaque is an essential CO-FACTOR — drug-influenced enlargement is markedly worse in inflamed, plaque-covered tissue, and meticulous hygiene substantially reduces its severity. This is why hygiene therapy is the first-line management.
• C. CORRECT — this is the EXCEPTION. PRIMARY HERPETIC GINGIVOSTOMATITIS is an ACUTE, self-limiting VIRAL infection presenting with fever, malaise, lymphadenopathy and multiple painful vesicles that rupture into ulcers, typically in young children. It resolves in 10–14 days and does NOT produce chronic fibrous gingival ENLARGEMENT.
• D. IS a cause, and the principal one. Phenytoin is the classic drug cause of gingival hyperplasia, affecting up to half of users, through increased fibroblast proliferation and collagen accumulation.

Note: The three drug classes causing gingival overgrowth: anticonvulsants (phenytoin), calcium channel blockers (nifedipine, amlodipine) and immunosuppressants (cyclosporine). Management is plaque control first, then gingivectomy if needed, with consideration of a medication change in consultation with the physician.