A baby boy is delivered vaginally after an uneventful 39-week pregnancy and a short labor. His birthweight is 2,500 g and Apgar score at 5 minutes is 10. His mother received comprehensive prenatal care, and findings on routine fetal sonograms were reported as normal. Specifically, no anomalies of the urinary tract or of the brain were detected, and a four-chamber projection of the heart is reported as normal. In the delivery room, the baby is normally reactive and cries. His heart rate is 120 beats/min, and his peripheral pulses are symmetric. All other physical findings are normal.

At 3 days of age, nursing vigorously and demonstrating a normal pattern of activity, the baby is about to go home. However, the neonatologist notes mild icterus as well as an inconsistent slight bluish discoloration of the lips. On auscultation, a grade 1–2/6 ejection murmur is heard at the mid-left sternal border, and...

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