7 day old baby presented in emergency department with unconsciousness, cyanosis and 85% oxygen saturation. The diagnosis – a) Tetralogy of fallotb) TGAc) TAPVCd) PDA

**Core Concept**
Tetralogy of Fallot (TOF) is a congenital heart defect characterized by four key features: ventricular septal defect (VSD), pulmonary stenosis, right ventricular hypertrophy, and an overriding aorta. This condition leads to decreased oxygenation of blood and can cause cyanosis, especially in infants.

**Why the Correct Answer is Right**
In TOF, the right ventricular outflow tract is obstructed, resulting in reduced blood flow to the lungs and subsequently decreased oxygenation of the blood. This leads to cyanosis, which is a key feature of the condition. The VSD allows deoxygenated blood from the right ventricle to mix with oxygenated blood from the left ventricle, further contributing to cyanosis. The overriding aorta allows blood to be shunted from the right ventricle to the systemic circulation, exacerbating cyanosis.

**Why Each Wrong Option is Incorrect**
**Option B:** Transposition of the Great Arteries (TGA) is a congenital heart defect where the two main arteries that carry blood out of the heart are reversed. This condition can cause cyanosis, but it is not characterized by the four features of TOF. TGA typically presents with severe cyanosis in the first few days of life.

**Option C:** Total Anomalous Pulmonary Venous Connection (TAPVC) is a congenital heart defect where the pulmonary veins do not connect normally to the left atrium. This condition can cause cyanosis, but it is not characterized by the four features of TOF. TAPVC typically presents with respiratory distress and failure to thrive.

**Option D:** Patent Ductus Arteriosus (PDA) is a congenital heart defect where the ductus arteriosus, a blood vessel that connects the aorta and pulmonary artery in the fetus, fails to close after birth. This condition can cause respiratory distress and failure to thrive, but it is not typically associated with cyanosis.

**Clinical Pearl / High-Yield Fact**
In infants with unexplained cyanosis, a careful examination of the chest and cardiac auscultation should be performed to identify the characteristic murmurs associated with TOF, such as a harsh systolic ejection murmur.

**Correct Answer:** a) Tetralogy of Fallot.