How can the scribes maintain that Christ is the son of David? There are three things that immediately jumps at the reader of today's Gospel passage: the passage is short, appears to be disconnected from the passage that immediately precedes it, and is difficult to understand. The passage is constituted of Jesus’ rebuttal of (and/or questioning of) what the scribes seem to know and teach about the Christ: that the Christ is the son of David. Whereas it is safe to say that the scribes, as were other religious leaders, were not on Jesus’ good books, teaching that the Christ was the son of David wasn’t their own invention. It was not something that they came up with in order to harm Jesus’ reputation. Whether figuratively or biologically, the Scriptures had always identified the Christ as a son of David. Jesus himself, we learn from the genealogy accounts in Luke and Matthew, list David as among Jesus’ ancestors. Isn’t Jesus described as the scion that stems of the stump of Jesse, David's father? So why was Jesus questioning the scribes’ assertion that the Christ is the son of David?
I would tend to believe that the answer to this question is found in the events that take place earlier on, right from the eleventh chapter of Mark's Gospel. Jesus had put himself in the limelight by his teachings and actions which seemed to corroborate the general feeling among the populace that he was the long awaited Messiah. This recognition by the people put him on a collision path with the scribes and other religious leaders. At one time, they had even questioned his authority. And even though Jesus had silenced them on the issue of their authority, they still had a bone to pick with him. They upped their game and moved to phase two of their attack on him: they wanted to discredit and label him an impostor. 
The religious leaders knew from their literature that the Christ had to descend from David. They knew Jesus was not descended from David. They knew his mother and father, and none descended from David (both Matthew and Luke link Jesus to David through Joseph, but David was Jesus' father only by adoption). Arming themselves with this knowledge, it appears they confronted Jesus: "We know the Christ HAS to descend from David. You do not descend from David. Ergo, You can't be the Christ.! Maybe it is this that puts Jesus on the offensive. He doesn’t dispute that he is not descended from David by blood. But that is no problem. He is greater than David. He sits at the right hand of the Father, and even David addresses him as Lord.