Differential cyanosis, also known as differential bruising, is most commonly seen in children with congenital heart disease such as ductus arteriosus. As the congenital heart disease progresses, the child develops mild cyanosis in the left upper extremity and normal cyanosis in the right upper extremity and cyanosis in the lower half of the body. This condition, in which both lower extremities are more severe than both upper extremities and the left upper extremity is more severe than the right upper extremity, is known as differential cyanosis. When, for some reason, arterial ductus arteriosus develops, 70% of the child’s left ventricular output passes through the ductus arteriosus into the pulmonary artery, and this direct flow of blood from the aorta into the pulmonary artery is also known as a left-to-right shunt. This direct flow of blood from the aorta into the pulmonary artery is also known as a left-to-right shunt. In the long run, pulmonary hypertension develops. When the pulmonary artery pressure exceeds the aortic pressure, the blood flow from the pulmonary artery is shunted into the descending aorta in the reverse direction, and the child presents with differential cyanosis, which is characterized by mild cyanosis of the left upper extremity, while the right upper extremity is normal, and cyanosis of the lower half of the body is more severe than that of the upper extremity. The etiology of differential cyanosis is complex. If you feel unwell, please consult the doctor immediately and follow the standardized treatment prescribed by the doctor.