The cluster on b3 and c4 is the kernel of a discovered check; White will attack Black's king with any move of his knight out of the bishop's path. Since the knight is on a light square, look for enemy pieces also on light squares that might make good targets. You thus find the Black rook on a8, the Black bishop on b7, and the Black knight on g4. The rook is the least attractive of these possibilities because it's guarded. As between Black’s knight and bishop, the knight makes the better target because White can pick up a pawn along the way: 1. Nxe5+, 2. Kh8; and now NxN.