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?

--> 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 --> d/t mixing of oxygenated blood with deoxygenated blood. PATENT DUCTUS AERIOSUS : --> Failure of closure of Ductus aeriosus after bih , resulting in persistent connection b/w 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.