An imperforate pulmonary valve. When associated with a VSD (variant of tetralogy of Fallot), pulmonary blood flow arises from aorto-pulmonary collaterals, and systemic venous return exits the right heart via the VSD. When associated with intact interventricular septum, pulmonary artery blood supply is via a patent ductus arteriosus, and systemic venous return exits the right heart via an obligatory ASD.