There is a bridge in the middle and two blocks opposite to each side of the bridge. Have you moved these blocks in place appropriately? There have been two marked spots on the ground, the blocks must be moved onto these spots, elase you'll be stuck later.
Use the ladders on the wall to reach the alcove from where you can jump to the first block. Then jump to the bridge, to the second block and finally into the alcove on the other side. So you have crossed the lava and now you should be able to reach the exit.