**Question:** Commonest congenital heart disease is:

A. Tetralogy of Fallot
B. Transposition of great arteries
C. Patent ductus arteriosus
D. Atrial septal defect

**Core Concept:** Congenital heart diseases (CHDs) are heart defects present at birth, resulting from abnormal development of the heart during fetal life. These diseases can be broadly categorized into four types: cyanotic, acyanotic, single ventricle, and double outlet right ventricle.

**Why the Correct Answer is Right:**

D. Atrial septal defect (ASD) is the commonest congenital heart disease among the types mentioned. ASD is characterized by an abnormal connection between the right and left atria, allowing blood to mix before being pumped to the body. In ASD, the heart's oxygenation remains normal, hence it is an acyanotic defect. ASD is the most prevalent type of cyanotic heart disease, which is further supported by D being the correct answer.

**Why Each Wrong Option is Incorrect:**

A. Tetralogy of Fallot (TOF) is a cyanotic heart disease involving the right ventricle, pulmonary artery, and aorta. It is not the commonest type of congenital heart disease.
B. Transposition of the great arteries (TGA) is a severe cyanotic heart disease, where the major blood vessels carrying oxygenated and deoxygenated blood are switched. This is not the most common type of congenital heart disease.
C. Patent ductus arteriosus (PDA) is a cyanotic heart disease caused by the persistence of the fetal ductus arteriosus, allowing deoxygenated blood to bypass the lungs and enter the systemic circulation directly. It is not the most common type of congenital heart disease.

**Clinical Pearl:**

Understanding the different types of congenital heart diseases is essential for proper diagnosis and management. ASD represents the most common type of cyanotic heart disease, and by knowing this, medical students can differentiate it from other congenital heart diseases and provide accurate treatment plans accordingly.