<prompt> A 4 year old child had cyanosis only involving the lower limbs, but not involving the upper limbs. Which of the following could be a possible underlying cause for differential cyanosis?PDA with reversal of shuntVSD with Eissenmenger syndromeTetralogy of FallotASD with reversal of shunt  <response>: -- Differential cyanosis is seen in PDA with reversal of shunt -- Differential cyanosis: No cyanosis in upper limbs -- as oxygenated blood goes there. Cyanosis in lower limbs -- dt mixing of oxygenated blood with deoxygenated blood. PATENT DUCTUS AERIOSUS : -- Failure of closure of Ductus aeriosus after bih , resulting in persistent connection bw aoa pulmonary aery. -- Hypoxia prematurity predispose to PDA -- AUSCULTATION -- Continuous machinery murmur is seen -- DOC for medical closure of PDA in preterm neonate -- PG Inhibitor -- Normally PDA is acyanotic hea disease but with reversal of shunt, differential cyanosis is seen. 