Even though a perfect price discriminator can extract all of the consumer surplus, how can it be efficient?

What will be an ideal response?

Efficiency has nothing to do with who gets the surpluses; that is, efficiency has nothing to do with whether the consumer surplus (or producer surplus) is large or small. Instead, a market is efficient as long as there is no deadweight loss. Because there is no deadweight loss with perfect price discrimination, even though the amount of consumer surplus is as small as possible—zero!—and the producer surplus is as large as possible, nonetheless the market is efficient.