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A 55-year-old man presented with a painless mass of progressive size on his right Achilles tendon. Over the years, several painful attacks are known on other joints.
He denied injuries but reported a history of recurrent gouty arthritis affecting the first metatarsophalangeal joint. On examination, the mass was firm, non-tender, and measured 3,2 cm in diameter.
What is the most likely diagnosis?
