The Eastern Question is an event that will happen to all European great powers, once 
Russia takes the decision Defender of the Eastern Christendom. It is part of an event chain, that can potentially lead to the Crimean War.
Russia has declared itself the defender of 
Orthodox Christians in The 
Ottoman Empire. That title gives Russia 0.1 prestige pr. month for the rest of the game if not challenged.
Every European great power can then choose to accept the title or deny it. Accepting it will remove 5 prestige. Denying it will give 5 prestige and improve relations with the Ottoman Empire by 30. It will also give Russia the event Protectorate Denied!.
In this following event, Russia can choose to back down at the cost of 20 prestige or keeping the title anyway, which gives the other great power in question a humiliate casus belli. If Russia chooses to keep the title anyway, you will lose the 5 prestige, you have gained from denying the title.
If Russia ever loses a war to a great power, that have denied the title, it will lose the title of Protector of Eastern Christendom and the prestige following it.