Which congenital heart defect is associated with lithium exposure in utero?

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Lithium exposure during the first trimester is classically linked to Ebstein's anomaly, a malformation of the tricuspid valve. In this condition, the septal and posterior leaflets of the tricuspid valve are displaced toward the apex of the right ventricle, causing part of the right ventricle to become atrialized and leading to severe tricuspid regurgitation. This results in an enlarged right atrium and can be detected on fetal or neonatal imaging by abnormal tricuspid valve positioning. The association explains why this defect is the one linked to in utero lithium exposure, whereas the other options do not share this teratogenic connection.