Progression of which of the following abnormalities flattens the portal veins?

The correct choice relating to the flattening of the portal veins is due to portal hypertension. In conditions of portal hypertension, increased pressure in the portal venous system results in various changes, one of which includes the appearance of the portal vein on ultrasound. As pressure builds up, the portal vein may become flattened or compressed, which can be noted during a sonographic examination.

Chronically elevated pressures within the portal system can lead to several complications, including the development of collateral venous channels to relieve the pressure. This condition can be a result of various underlying issues, such as liver cirrhosis or hepatic vein obstruction, which further emphasize the importance of recognizing the changes in vascular structures like the portal vein.

While conditions like biliary obstruction can lead to gallbladder distention and other biliary system changes, they do not directly cause the flattening of the portal vein. Furthermore, ascariasis and pneumobilia pertain to different pathological mechanisms that do not typically affect the shape of the portal vein in the same context. Understanding these relationships is crucial for proper diagnosis and management in sonography.