In Chapter 11, Catherine compares David to an apple.What does she mean by this comparison?

Catherine compares David to a not so perfect apple..... she sees him as an apple that's perfect on the outside and damaged or imperfect on the inside..... he looks perfect, but inside he's broken, easily frightened. David suffers from autism, this comparison directly alludes to the fact that people suffering from autism look like everyone else.... their disability is internal, and not apparent in their physical appearance.