Final answer:
A paradoxical embolus causing a focal ischemic stroke typically requires the presence of a cardiac shunt such as a patent foramen ovale (PFO) or a defect in the interatrial or interventricular septum.
Step-by-step explanation:
A paradoxical embolus causing a focal ischemic stroke implies the presence of a heart condition that permits the abnormal passage of material from the venous to the arterial system. This typically requires a patent foramen ovale (PFO) or another type of cardiac shunt, such as a ventricular septal defect or atrial septal defect. In normal circumstances, a venous clot (embolus) would travel to the lungs leading to a pulmonary embolism. However, with a PFO or similar shunt, a venous embolus can bypass the lungs and enter systemic circulation, potentially causing an ischemic stroke if it lodges in a cerebral artery.