What cardiac anomaly occurs with the highest frequency in fetuses exposed to lithium in utero (especially during the first trimester?)

I'll stick a random photo here, in case you didn't want the answer right away. It was taken a long time ago, during a pre-medical school vacation to my old college alma mater, but looking at it now, it could pretty much be anywhere.



And now, back to work.
The answer is Ebstein anomaly, which is a malformation of the tricuspid valve. Specifically, it is a congenital malformation that is characterized by downward (apical) displacement of the septal and posterior tricuspid valve leaflets, as well as malformation and abnormal attachment of the anterior leaflet to the right ventricular free wall. In other words, the tricuspid valve between the right atrium and right ventricle did not develop appropriately and does not work properly, so blood can leak backwards and cause atrial enlargement and potential heart failure.

Because of the increased pressure of backflow blood into the right atrium, many people with Ebstein anomaly also have an atrial septal defect, which allows blood to go directly from the right atrium to left atrium, bypassing the right ventricle and lungs, thus sending deoxygenated blood into the circulation.

Depending on severity, Ebstein anomaly can be asymptomatic, or present with cyanosis, heart failure, dyspnea, tricuspid regurgitation, fatigue, arrhythmias, or many other signs and symptoms. Definitive diagnosis is by echocardiogram study, and the treatment varies from none to surgery (repair/replacement).