Leon Zhidovin

In 1490, the course of Russian history was in the hands of a Jewish doctor. It didn't go well.

Little is known about Leon Zhidovin... (Oh-oh, say our Russian-speaking readers. Zhidovin? Not a huge deal here, as the word "zhid" had not become a slur... yet. Of course, it could be used as a slur, because, you know, Russians and their love of Jews. Moving on.)

Leon was a Venetian doctor, the first physician from Western Europe to come to Muscovy. Ivan III's eldest son, also Ivan, was ill with "aches in the legs". (Probably gout, although we wouldn't bet against syphilis.) Leon tried to cure him and failed. (If this was gout, he was centuries away from effective treatment. If this was syphilis, well...)

At the time, the blame of failed medicine went squarely on the doctor (obviously, being Jewish didn't help), so Leon was hanged, ending his brief stay in Muscovy.

So instead of prince Ivan becoming Ivan IV, it was his younger brother Vasily who became czar after their father's death. Ivan IV did follow soon... He was Vasily's son, and is better known as Ivan the Terrible.

So if Leon found a way not to cure, but mitigate the prince's gout (or syphilis), it's quite possible that Russian history would go down a completely different path. It wouldn't make it less abominable or antisemitic, just... different.