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What is MC cause of death in an adult with PDA is – and explain in detail?
A most common cause of death in PDA is CCF.
A second most common cause of death is infective endocarditis.

Course and complications of PDA

Spontaneous closure of PDA does not usually occur in full-term infants because PDA in term infants results from a structural abnormality of ductal smooth muscles (In premature infants spontaneous closure may occur because PDA in these infants results from unresponsiveness to oxygen).
Pulmonary vascular pressure regress slower than normal and maximum decrease occurs around 6-8 weeks (normally at 1-3 weeks).

Common complications are : -

CHF → MC complication and MC cause of death       
Infective endocarditis
Pulmonary hypertension 
Pneumonia
Aneurysm of PDA → Rare and possible rupture in adult life