What is the actual defect with body dysmorphic disorder ?

There is none: The obsession with a perceived minor flaw is just that--a perception not a reality.

Unknown: There is no single cause or defect in a person who has body dysmorphic disorder; research shows that a number of factors may be involved and that they can occur in combination. Bdd can be associated with eating disorders, such as compulsive overeating, anorexia nervosa or bulimia, or it can be more of a phobia, associated instead with social phobia or social anxiety disorder.