A patient has a bluish discoloration in the retromolar area. All of the following are reasonable differential diagnoses EXCEPT:

Correct answer: C. Fibroma

• A. IS a valid differential. A varix is a dilated vein, most typically on the ventral tongue or lower lip in older patients, appearing as a soft bluish-purple swelling that BLANCHES with diascopy — the key clinical test.
• B. IS a valid differential. A blue nevus is a benign melanocytic proliferation deep in the connective tissue; the depth of the melanin produces the blue appearance through the Tyndall effect. The palate is the most frequent oral site, but it occurs elsewhere.
• C. CORRECT — this is the EXCEPTION. A FIBROMA is a firm, PINK or normal-coloured, dome-shaped fibrous overgrowth arising from chronic irritation. It contains no pigment and no vascular component, so it does not present as a bluish lesion. Colour is the discriminator here.
• D. IS a valid differential. An amalgam tattoo is the most common cause of localised blue-grey pigmentation in the mouth, and the retromolar/edentulous ridge region — where amalgam-restored teeth were extracted — is a classic site. Radiographs may show radiopaque particles.

Note: Diascopy (pressing with a glass slide) is the quick bedside discriminator: vascular lesions blanch, pigmented lesions such as nevi, melanoma and amalgam tattoo do not.