In a piece of forensic medical detective work an American doctor has diagnosed Michelangelo's condition from a fresco by Raphael that adorns the Pope's library, the Stanza della Signatura, in the Vatican.
Carlos Hugo Espinel, a specialist in blood-pressure disorders at Georgetown University, Washington, also suggests in an article in The Lancet that Michelangelo's obsession with his work and neglect of his diet may have caused the problem. Dr Espinel cites Vasari, the writer and painter, who observed that the master would go for days on bread and wine.
Wine, notes Dr Espinel, was processed in lead containers, and Michelangelo may also have been exposed to lead-based paints. Plumbism, or lead poisoning, could explain the gout. The key to the diagnosis is the central figure in Raphael's School of Athens.
The fresco portrays luminaries of pre-Renaissance thought, including Plato holding his Timaeus, Aristotle with his Ethics and Diogenes reclining on the steps. One man, in the centre foreground, remains unidentified. Dr Espinel believes it is Michelangelo.
There are several clues. The figure is not included in cartoons Raphael drew in preparation for the fresco, suggesting he was a late addition. He is in contemporary clothes, a simple purple cassock and boots, while the rest are in classical robes.
Of the Sistine Chapel's 300-plus figures, that of Jeremiah, one of Michelangelo's alleged self-portraits, has a hint of boots. Vasari described him wearing "buskins of dogskin."
The man is of the right age and size and, although his head is turned, his nose appears to be twisted, in keeping with Vasari's description that it was "somewhat flattened, broken in his youth by Torregiano's fist".
But why was Michelangelo included? Dr Espinel suggests that Raphael may have had an early glimpse of the Sistine chapel when Michelangelo allowed the first viewing on 14 August 1511.
One glance would have persuaded him that Michelangelo belonged with the greatest geniuses who had lived.
"Scanning the divine images above, realising that now he would have to change and the world would have to change, Raphael may have decided to add a portrait of Michelangelo. The style of this portrait, the figure's space, mass and perspective are ... more akin to the images in the Sistine chapel."
The seated figure has his right knee framed and highlighted by his own shadow, with his boots rolled down. Dr Espinel writes: "It is not a hot knee: its skin, pale and tense, show no sign of inflammation, nor is there ulceration. But underneath the skin there are lumps, knobs of a light yellow that appear dense and hard."
In his writings Michelangelo described episodes of bladder and kidney problems that persisted throughout his life.
In one poem he laments "I'ho `imparato conoce l'orina" [I have learnt to know the urine] and later he recorded in a letter how he had passed "gravel" and fragments of a kidney stone.
Putting this together with his knobbly knee, Dr Espinel says there is only one possible diagnosis. Although written descriptions of the condition are common throughout history, illustrations are rare. Michelangelo's knee, as portrayed by Raphael, can take its place in medical textbooks, suggests Dr Espinel, as an early pictorial description of gout.