A patient presents with a 1 cm x 1 cm blue-purple pigmented lesion on the retromolar pad. The patient has a history of extensive amalgam restorations in the area. The lesion is flat, painless, and does not blanch with pressure. What is the most likely diagnosis?

An amalgam tattoo (focal argyrosis) is the most common pigmented lesion of the oral mucosa. It presents as a flat, blue-gray or blue-purple pigmented macule, most common on the gingiva, alveolar mucosa, or retromolar pad — areas near amalgam restorations. It is asymptomatic and does not blanch with pressure (unlike vascular lesions). The history of adjacent amalgam restorations is the key diagnostic clue. Radiographic evidence of metallic particles may be visible. Biopsy is recommended if the diagnosis is uncertain, particularly to rule out melanoma. Blue nevus is intradermal melanocytic, typically found on the hard palate. Melanoma would be irregular, rapidly enlarging, and not correlated with amalgam.