## Core Concept
Budd-Chiari syndrome is a rare condition characterized by the obstruction of hepatic veins or the inferior vena cava, leading to liver dysfunction. The initial investigation of choice should ideally be non-invasive, readily available, and capable of providing rapid diagnosis. **Imaging techniques** play a crucial role in the diagnosis of Budd-Chiari syndrome.

## Why the Correct Answer is Right
The correct answer, **Ultrasound**, is the initial investigation of choice in acute Budd-Chiari syndrome because it is **non-invasive**, **widely available**, and can quickly demonstrate signs suggestive of hepatic venous outflow obstruction, such as **hepatic vein thrombosis** or **stenosis**, and **ascites**. Doppler ultrasound can also assess blood flow through the hepatic veins and inferior vena cava.

## Why Each Wrong Option is Incorrect
- **Option A:** CT abdomen with contrast is a sensitive and specific diagnostic tool but is not the initial investigation of choice due to its higher cost, limited availability compared to ultrasound, and the use of contrast.
- **Option B:** MRI abdomen is highly sensitive and specific for diagnosing Budd-Chiari syndrome but is not used as an initial investigation due to its limited availability, higher cost, and longer examination time compared to ultrasound.
- **Option D:** Liver biopsy is an invasive procedure and is not suitable as an initial investigation. It may be considered later for confirming the diagnosis or assessing the extent of liver damage.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that Budd-Chiari syndrome can present acutely with **severe abdominal pain**, **ascites**, and **liver dysfunction**. Early diagnosis with **ultrasound** can guide further management, including ** anticoagulation**, **thrombolysis**, or **interventional radiology procedures** to relieve the obstruction.

## Correct Answer Line
**Correct Answer: C. Ultrasound**.