A 3-week-old boy is brought to the physician because his lips turn blue when he cries. He feeds poorly and sweats while feeding. He is below the 5th percentile for both height and weight. There is cyanosis of the tongue and lips. Heart sounds are normal. A grade 3/6 systolic ejection murmur is heard over the left upper sternal border. An x-ray of the chest shows a small boot-shaped heart and decreased pulmonary vascular markings. Further evaluation of this patient is most likely to show which of the following findings?