Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the symptoms of Tetralogy of Fallot ?
    ### Response:
    Cyanosis is an important sign of tetralogy of Fallot. Cyanosis is a bluish tint to the skin, lips, and fingernails. Low oxygen levels in the blood cause cyanosis.
                
Babies who have unrepaired tetralogy of Fallot sometimes have "tet spells." These spells happen in response to an activity like crying or having a bowel movement.
    