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What is A 7 year old girl from Bihar presented with three episodes of massive hematemesis and melena. There is no history of jaundice. On examination, she had a large spleen, non-palpable liver and mild ascites. Portal vein was not visualized on ultrasonography. Liver function tests were normal and endoscopy revealed esophageal varices. The most likely diagnosis is – and explain in detail?
Differentiation between Extrahepatic and Hepatic / Post hepatic portal hypertension 

Post sinusoidal and hepatic causes of portal hypertension will have
evidence of Liver parenchymal damage / deranged LFT (jaundice)
a dilated portal vein on ultrasonography
Extrahepatic presinusoidal obstruction will not have any effect on the Liver and hence there will be no evidence of hepatocellular damage.
No jaundice / No raised deranged LFT

Also non visualization of portal vein is a characteristic of portal vein thrombosis, an Extrahepatic cause.

Esophageal varices and splenomegaly are a manifestation of portal hypertension and will be present in all the above sites of obstruction.