Use the same thought process to find a pin for White. Which Black pieces are lined up with his king? His knight, on the same diagonal. What can White use to pin the knight? His queen, with Qa4. The knight can't be moved or guarded, so QxN wins it a move later.
A good question to ask about a pin like this is what Black might play in response after White starts with Qa4. Might he launch a counterthreat of his own? He can, with g5-g4; and now it looks like he is poised to take White’s knight if White takes his. But rather than fuss over this threat, White just goes ahead and plays QxN+. Notice that this gives check, a natural result when a piece pinned to its king is taken. Black therefore must spend his next move taking his king out of danger, rather than playing g4xN as he hoped—and then White will have a move he can use to relocate his knight safely. Here White took advantage of the principle known as the priority of check: QxN forced Black to address the threat to his king, and in effect gave White two moves in a row. We have seen this theme before and will return to it many more times.