A previously healthy 12-year-old boy is seen for a health supervision visit by his pediatrician. Clinical examination is significant for cyanosis of his lips, mucus membranes, and hands, and clubbing of fingers. He has not seen a primary care provider in the past 3 years. He is able to play, bike, be involved in physical education activities in school, and keep up with peers in physical activities without shortness of breath or fatigue. He reports self-limiting episodes of epistaxis once or twice a year for the past 2 to 3 years. He has not traveled outside the country. Family history is unrevealing. Pulse oximetry shows oxygen saturation of 75% to 80%. His heart rate is 70 beats/min, respiratory rate is 20 breaths/min, and blood pressure is 100/52 mm Hg in the right arm in a sitting position, with no difference...

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