What is a classic sign of Eisenmenger's syndrome?

Eisenmenger's syndrome is characterized by a right to left shunt that occurs as a result of long-standing pulmonary hypertension, typically following an initially left to right shunt like a ventricular septal defect or atrial septal defect. Over time, the increased pressure in the pulmonary circulation reverses the direction of blood flow, leading to cyanosis due to deoxygenated blood entering the systemic circulation.

Cyanosis is a hallmark sign of this condition, as it indicates that the body is not receiving an adequate supply of oxygen. Patients may exhibit symptoms such as clubbing of the fingers and toes and experience a range of systemic issues due to low oxygen levels. The right to left shunt leads to blood bypassing the lungs, where it would typically undergo oxygenation, thus making this option the defining feature of Eisenmenger's syndrome.