A five day old, full term male infant was severely cyanotic at bih. Prostaglandin E was administered initially and later ballooned atrial septosomy was done which showed improvement in oxygenation. The most likely diagnosis of this infant is :

Answer is B (Transposition of great vessels) Transposition of Great vessels is a condition chracterized by aoa arising from the right ventricle and pulmonary aery from the left ventricle. This results in complete separation of systemic and pulmonary circulation. Since systemic and pulmonary circulations are separate, survival depends on mixing of oxygenated pulmonary blood and deoxygenated systemic blood which can Occur' At Atrial level - Through ASD At Ventricle level - Through VSD At Great vessel level - Through Patent ductus aeriosus Oxygenation is normal in foetus but after bih as ductus aeriosus closes severe hypoxia occurs and in the absence of V.S.D, or ASD child presents with cyanosis and tachypnea at bih.