If marcy is feeling guilty about lying to her mother, supporters of freud's psychoanalytic theory would most likely argue that her is making her feel this way.

The answer is superego.
In our mind, our superego played the criticizing and the moralizing role.
When marcy lied to her mother, her superego question her personal morality because of her actions and caused an emotional distress inside marcy's mind which tend to prevent her from repeating that action again in the future.