**Core Concept**
Budd Chiari syndrome is a rare condition characterized by occlusion of the hepatic veins, leading to liver congestion, ascites, and abdominal pain. This condition is often associated with thrombosis of the hepatic veins, which can be caused by various factors including coagulopathy, hypercoagulable states, and other underlying medical conditions.

**Why the Correct Answer is Right**
The correct answer, **D. Thrombosis of hepatic veins**, is the most common cause of Budd Chiari syndrome. This is because thrombosis of the hepatic veins leads to obstruction of blood flow from the liver to the inferior vena cava, resulting in liver congestion, ascites, and abdominal pain. The hepatic veins are particularly susceptible to thrombosis due to their anatomy and the high velocity of blood flow through them.

**Why Each Wrong Option is Incorrect**
**Option A:** Idiopathic Budd Chiari syndrome refers to cases where the underlying cause is unknown. While idiopathic cases do exist, they are not the most common cause of Budd Chiari syndrome.

**Option B:** Valves in hepatic veins can contribute to Budd Chiari syndrome, but they are not the most common cause. The hepatic veins do not have valves, and any abnormal valves would likely be a result of the underlying thrombosis.

**Option C:** Hepatocellular carcinoma can cause Budd Chiari syndrome through invasion of the hepatic veins, but it is not the most common cause. Tumors can lead to thrombosis of the hepatic veins, but this is a secondary effect rather than the primary cause.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Budd Chiari syndrome is often associated with coagulopathy, hypercoagulable states, and other underlying medical conditions. A high index of suspicion and a thorough evaluation of the patient's coagulation status and medical history are crucial in diagnosing and managing this condition.

**✓ Correct Answer: D. Thrombosis of hepatic veins**