Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Tetralogy of Fallot ?
    ### Response:
    Tetralogy (teh-TRAL-o-je) of Fallot (fah-LO) is a congenital heart defect. This is a problem with the heart's structure that's present at birth. Congenital heart defects change the normal flow of blood through the heart.
                
Tetralogy of Fallot is a rare, complex heart defect.
    