**Core Concept**
Budd Chiari syndrome is a rare disorder characterized by the obstruction of the hepatic veins or the inferior vena cava, leading to liver enlargement, ascites, and abdominal pain. The condition occurs due to thrombosis or compression of the hepatic veins, resulting in increased pressure within the hepatic sinusoids and congestion of the liver.

**Why the Correct Answer is Right**
The correct answer is hepatic veins because the hepatic veins are the primary site of venous thrombosis in Budd Chiari syndrome. The hepatic veins are responsible for draining blood from the liver into the inferior vena cava. Thrombosis of the hepatic veins leads to increased pressure within the liver, causing liver enlargement, ascites, and abdominal pain. This is due to the backflow of blood into the sinusoids, causing congestion and subsequent liver dysfunction.

**Why Each Wrong Option is Incorrect**
**Option A:** Infrahepatic inferior vena cava is incorrect because while thrombosis of the inferior vena cava can occur in Budd Chiari syndrome, it is not the primary site of thrombosis. Thrombosis of the inferior vena cava typically occurs below the level of the hepatic veins.
**Option B:** Infrarenal inferior vena cava is incorrect because thrombosis of the inferior vena cava below the level of the renal veins is not characteristic of Budd Chiari syndrome.
**Option D:** Portal veins are incorrect because Budd Chiari syndrome is characterized by thrombosis of the hepatic veins, not the portal veins. Thrombosis of the portal veins leads to a different condition known as portal vein thrombosis.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that Budd Chiari syndrome is often associated with hypercoagulable states, such as protein C or protein S deficiency, and should be considered in patients with unexplained liver enlargement and ascites.

✓ Correct Answer: C. Hepatic veins