Pylephlebitis is an inflammation

Inflammation

Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

 of the portal vein or any of its branches. It is usually a complication of intraabdominal sepsis, most often following diverticulitis

Diverticulitis

Diverticulitis is a common digestive disease particularly found in the large intestine. Diverticulitis develops from diverticulosis, which involves the formation of pouches on the outside of the colon...

 or appendicitis

Appendicitis

Appendicitis is a condition characterized by inflammation of the appendix. It is classified as a medical emergency and many cases require removal of the inflamed appendix, either by laparotomy or laparoscopy. Untreated, mortality is high, mainly because of the risk of rupture leading to...

. It has a poor prognosis. It typically presents with fever, rigors and jaundice.

This can lead to pre-hepatic portal hypertension due to the obstruction.

It is a cause of portal hypertension.

In a third trimester pregnant woman, there is a minimal risk of premature delivery; however, if rupture of the appendix occurs, peritonitis can lead to fetal death, abscess formation and pylephlebitis (infectious thrombosis of the portal veins).Pylephlebitis is especially feared in pregnant women because of the already existing prothrombotic state. These complications are rare but feasible, especially if therapy is not offered within the first 24 hours of the initiation of symptoms. (Dr. Suleiman, Y)
Questions like this are classic for USMLE step 3.