Explanation

Cyanosis in the tetralogy of Fallot is primarily due to the combination of the four defects, with pulmonary stenosis playing a crucial role.

Pulmonary stenosis restricts blood flow to:

• the lungs
• increasing pressure in the right ventricle 
• leading to a right-to-left shunt through the ventricular septal defect

This shunt allows deoxygenated blood to enter the systemic circulation, causing cyanosis.