Budd-Chiari syndrome is caused by blood clots that completely or partially block blood flow from the liver. A blockage may occur in the small or large veins that carry blood from the liver (hepatic veins) or the inferior vena cava (the large vein that carries blood from the lower part of the body, including the liver, to the heart). It may occur gradually or suddenly. In which of the following circumstances does a blockage tend to occur suddenly?