A 3-yr-old girl has a 2-wk history of fever associated with bifrontal headache, lethargy, and vomiting. She has a history of perioral cyanosis and dyspnea with exertion beginning in infancy. She suddenly has a 10-min focal tonic-clonic seizure. The child is obtunded and has a temperature of 100.8°F (38.2°C), pulse of 118/min, and blood pressure of 96/70 mm Hg in her right arm, measured while she is supine. Perioral cyanosis is noted at rest. A harsh pansystolic murmur is heard best along the left sternal border. Examination of her eye grounds reveals bilateral papilledema. She has right-sided weakness associated with hyperreflexia and an extensor plantar reflex. The most likely cause of the hemiparesis is: