Absence of the right superior vena cava (RSVC). An absent RSVC is always associated with a persistent left superior vena cava (PLSVC). During normal fetal development, the left-sided anterior venous cardinal system regresses, leaving the coronary sinus (CS) and the ligament of Marshall. Failure of the closure of the left anterior cardinal vein results in PLSVC. In general, PLSVC is associated with the right superior vena cava (RSVC) and drains into the RA via a dilated CS. When developmental arrest occurs at an earlier stage, the CS is absent and the PLSVC drains into the left atrium. [from
HPO]