**Core Concept**
Post-hepatic portal hypertension refers to increased pressure in the portal vein due to obstruction or thrombosis of the hepatic veins or inferior vena cava. This condition leads to a reversal of blood flow in the portal vein, causing esophageal and gastric varices.

**Why the Correct Answer is Right**
The correct answer is related to the obstruction of the hepatic veins or inferior vena cava, which leads to a significant increase in pressure in the portal vein. This obstruction can be due to various causes, including hepatic vein thrombosis, Budd-Chiari syndrome, or inferior vena cava obstruction. The resulting post-hepatic portal hypertension can cause severe complications, including ascites, hepatic encephalopathy, and gastrointestinal bleeding.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because pre-hepatic portal hypertension is caused by increased resistance to blood flow in the portal vein, typically due to splenic vein thrombosis or portal vein thrombosis, but not due to obstruction of the hepatic veins or inferior vena cava.
* **Option B:** This option is incorrect because intrahepatic portal hypertension is caused by increased resistance to blood flow within the liver, typically due to cirrhosis or fatty liver disease, but not due to obstruction of the hepatic veins or inferior vena cava.
* **Option C:** This option is incorrect because extrahepatic portal hypertension is caused by increased resistance to blood flow outside the liver, typically due to splenic vein thrombosis or portal vein thrombosis, but not due to obstruction of the hepatic veins or inferior vena cava.

**Clinical Pearl / High-Yield Fact**
Budd-Chiari syndrome, a condition characterized by hepatic vein thrombosis, is a classic cause of post-hepatic portal hypertension. This condition can be diagnosed using imaging studies such as ultrasound or CT scan, and treatment typically involves anticoagulation therapy and liver transplantation.

**Correct Answer: D. Obstruction of the hepatic veins or inferior vena cava.**