Final answer:
The highest-priority intervention for an infant with an unrepaired tetralogy of Fallot defect becoming extremely cyanotic during a routine blood draw would be to administer 100% oxygen by blow-by. This intervention helps to increase the oxygen supply and reduce cyanosis. The second-priority intervention would be to place the infant in a knee-chest position.
Step-by-step explanation:
The highest-priority intervention for an infant with an unrepaired tetralogy of Fallot defect becoming extremely cyanotic during a routine blood draw would be to administer 100% oxygen by blow-by. This intervention helps to increase the oxygen supply and reduce cyanosis. The second-priority intervention would be to place the infant in a knee-chest position. This position helps to improve blood flow to the lungs and reduce the cyanotic episode. The lowest-priority intervention would be to give morphine subcutaneously or by an existing intravenous line. Morphine is not typically used as a first-line treatment for cyanotic episodes, and other measures should be prioritized.