Final answer:
The nurse should immediately administer oxygen to an infant with tetralogy of Fallot experiencing a hypercyanotic spell, as this action increases the oxygen available in the bloodstream and alleviates symptoms of cyanosis.
Step-by-step explanation:
When an infant with tetralogy of Fallot begins to have a hypercyanotic spell, the most immediate action that a nurse should take is C) Administer oxygen. This is because the spell is characterized by a sudden onset of cyanosis, or blue discoloration of the skin, due to decreased oxygen in the bloodstream. In infants with congenital heart defects like tetralogy of Fallot, administering oxygen can help to alleviate this hypoxic episode by increasing the amount of oxygen available to the body.
It is important to note that while elevating the infant's legs can help in certain circumstances by increasing venous return to the heart and thus cardiac output, this is not the initial action to take during a hypercyanotic spell. A diuretic is not appropriate because it would not directly address the immediate need for increased oxygen. Although providing a pacifier for comfort may be helpful in general care, it is not a therapeutic response to a hypercyanotic spell, which requires immediate medical intervention.