**Question:** Which one of the following is a cyanotic congenital heart disease?

A. Tetralogy of Fallot
B. Patent ductus arteriosus
C. Atrial septal defect (ASD)
D. Ventricular septal defect (VSD)

**Core Concept:**
Cyanotic congenital heart diseases are a group of disorders characterized by the shunting of deoxygenated blood from the right ventricle to the systemic circulation, leading to cyanosis (blue discoloration of mucous membranes). Cyanotic congenital heart diseases primarily affect the right side of the heart, and the oxygen saturation of deoxygenated blood decreases.

**Why the Correct Answer is Right:**
Tetralogy of Fallot (Option A) is a cyanotic congenital heart disease characterized by a large ventricular septal defect (VSD), a right ventricular outflow tract obstruction, a pulmonary artery stenosis, and a right ventricular hypertrophy. This condition results in the shunting of deoxygenated blood from the right ventricle to the systemic circulation, leading to cyanosis.

**Why Each Wrong Option is Incorrect:**

B. Patent ductus arteriosus (Option B) is a condition in which the ductus arteriosus, a fetal shunt, remains open after birth, causing a connection between the pulmonary artery and the aorta. While a patent ductus arteriosus can cause hypoxia, it is not a cyanotic congenital heart disease as it does not involve the right side of the heart.

C. Atrial septal defect (ASD) (Option C) is a congenital heart defect characterized by a hole in the atrial septum. ASD is a type of ASD (Atrial Septal Defect) and does not involve the right ventricle or right-sided heart structures, making it a non-cyanotic congenital heart disease.

D. Ventricular septal defect (VSD) (Option D) is a congenital heart defect characterized by a hole in the ventricular septum. VSD is a type of ASD (Atrial Septal Defect) and does not involve the right ventricle or right-sided heart structures, making it a non-cyanotic congenital heart disease, similar to Option C.

**Clinical Pearl:**
Understanding the anatomy and pathophysiology of congenital heart diseases is essential for diagnosing and treating patients accurately. Cyanotic congenital heart diseases, like Tetralogy of Fallot, involve the right side of the heart and lead to cyanosis due to shunting of deoxygenated blood from the right ventricle to the systemic circulation.