A 9-year-old boy arrived at a heart institute in Mexico City with blue-tinged fingers, drumstick-shaped fingertips, and an oxygen saturation of 86 percent on room air. He had been blue and clubbed since infancy. An outside echocardiogram had found a hole between his ventricles, but the imaging window was poor, and nobody could see what else was going on in there.
An MRI scanner finally showed them. The tip of his right ventricle had never formed. His left ventricle had stretched sideways and elongated toward the right, occupying the space where the right ventricular apex should have been and producing a silhouette the authors named the Christmas hat sign.
