It's the etrog's turn - one that is stolen or dry cannot be used for the 4 species. Plus, the various regulations that apply to the etrog as a fruit (terumot and maaserot, specifically). Plus, different kinds of blemishes on the etrog itself that invalidate it for the 4 species (or not). Plus, the size parameters for the etrog. Also, why "pri etz hadar" is known to be the etrog. Also, how to handle "demai," to turn benefit of poor people.