If the three-vessel/trachea view shows a side-by-side arrangement of the great arteries, which diagnosis is most likely?

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The key idea is recognizing parallel, side-by-side great arteries in the three-vessel/trachea view. In a normal fetal heart, the pulmonary artery and aorta form a crisscross pattern as they course toward their outlets. In transposition of the great arteries, the outflow tracts are connected to the opposite ventricles, so the two great vessels run parallel rather than crossing. This side-by-side configuration is the hallmark seen on the 3V view for TGA. Other defects produce different patterns: truncus arteriosus shows a single arterial trunk, hypoplastic left heart often has a very small aorta, and tetralogy of Fallot typically alters the size relationship of the great vessels but not a parallel two-vessel appearance. So, the side-by-side arrangement on the 3V view points to transposition of the great arteries.