Leonardo da Vinci left the Mona Lisa unfinished because he gravely injured his arm while fainting, a new study argues.
The cause of the renaissance artist's disability has been debated by art historians for centuries, and in recent years partial paralysis as a result of a stroke has emerged as the dominant theory.
Proponents have pointed to da Vinic's vegetarianism as a clue, arguing that the high-dairy diet he is assumed to have eaten would have made a stroke more likely, The Telegraph UK reports.
However, two senior Italian doctors now claim to have solved the mystery, having studied a drawing of da Vinci by an obscure Lombard artist.
The blood-red chalk picture by Giovan Ambrogio Finio depicts an elderly da Vinci with his lower right arm at right-angles to his body, swaddled in folds of his clothes as if in a sling.
His thumb, first and second figures are extended, with his fourth and fifth fingers are contracted.
Writing in the Journal of the Royal Society of Medicine, Dr Davide Lazzeri, a plastic surgeon, and Dr Carlo Rossi, a neurologist, argue that if da Vinci had indeed suffered a stroke, it is far more likely his entire fist would have been clenched.
They also point out there is no evidence the old master suffered any facial or neurological impairment, which is common following a stroke.
Instead, the medics say the hand position, sometimes known as "claw hand", is far more likely to have been ulna palsy, the kind of traumatic nerve damage elderly people often sustain following a fall while fainting or suffering a dizzy spell.
"This may explain why he left numerous paintings incomplete, including the Mona Lisa, during the last five years of his career as a painter while he continued teaching and drawing," said Dr Lazzeri.
Da Vinici was unusual because he was left-handed, drawing and writing with his left hand, and yet it is widely agreed that he used his right hand to paint.
Drawings attributed to Da Vinci show the shading and hatching sloping from upper left to lower fright, leaving few doubts about his left-handedness, meanwhile biographical mainly concur that he used his right hand while painting.
According to the new study, this explains why the artist was lucid and active - which he might not have been following a stroke - but not able to paint in the latter years of his career.
It quotes an entry in the diary of Antonio de Beatis, assistant to Cardinal Luigi D'aragona, following a visit to da Vinci in 1517, which states: "One cannot indeed expect any more good work from him, as a certain paralysis has crippled his right hand...And although Messer Leonardo can no longer paint with the sweetness which was peculiar to him, he can still design and instruct others."
The Mona Lisa is thought to have been started in 1503 or 1504 in Florence, and da Vinci is said to have "lingered over it" for years before leaving it unfinished.
Albeit not his primary medium, the drawings created by Da Vinci following his arm injury occupy an important position in the cannon.
These include The Virgin and Child with St Anne, as well as studies of cats, horses, dragons and St George.
Before the injury, thought to have taken place in 1505, Da Vinci was reputed to have been immensely strong.
A passage from a book by fellow painter Giorgio Vasari, published three decades after the polymath's death, read: "he was physically so strong that he could withstand violence and with his right hand he could bend the ring of an iron door knocker or a horseshoe as if they were lead."