Portal Hypertension: This condition is the most likely diagnosis due to the presence of splenomegaly and a dilated portal vein, which are hallmark signs of increased pressure within the portal venous system. Portal hypertension can result from various causes, including cirrhosis, thrombosis, or schistosomiasis.

Cirrhosis: Often leads to portal hypertension, which in turn causes splenomegaly and dilation of the portal vein. Cirrhosis can be due to alcohol abuse, hepatitis B or C, non-alcoholic fatty liver disease, or other liver diseases.
Portal Vein Thrombosis: Can cause splenomegaly and dilation of the portal vein due to obstruction of blood flow, leading to increased pressure within the portal system.
Schistosomiasis: A parasitic infection that can cause portal hypertension and subsequent splenomegaly and dilation of the portal vein, particularly in endemic areas.

Budd-Chiari Syndrome: Although less common, this condition involves thrombosis of the hepatic veins, which can lead to acute liver failure and has a high mortality rate if not promptly diagnosed and treated.
Splenic Artery Aneurysm: While rare, it can rupture and cause severe bleeding, making it a critical diagnosis not to miss, especially in pregnant women or those with a history of liver disease.

Sarcoidosis: Can cause splenomegaly and may affect the liver, leading to portal hypertension, although this is a less common manifestation.
Lymphoma: Certain types of lymphoma can involve the spleen and liver, potentially causing splenomegaly and portal vein dilation, but this would be less common compared to other causes.
Congenital Portal Vein Anomalies: Rare conditions such as portal vein cavernous transformation or congenital portal vein atresia can lead to splenomegaly and dilation of the portal vein, often presenting in childhood.