This is the same as the previous problem but a move earlier. It is given here as a little study in how the same train of thought looks from a defensive perspective. It would be easy for White to imagine that he can afford to play a3xB since then Black's QxR is met with QxQ. But when Black’s knight is placed this dangerously it is unwise to initiate any exchanges without making sure that no forking opportunities will be created for it—as would be here.