**Core Concept**
The initial investigation for an amoebic liver abscess involves imaging and serological tests to confirm the diagnosis. Amoebic liver abscess is caused by Entamoeba histolytica, which invades the liver through the bloodstream, leading to an abscess formation. The primary goal of initial investigation is to confirm the diagnosis and rule out other causes of liver abscess.

**Why the Correct Answer is Right**
The correct answer is **Ultrasound**. Ultrasound is the preferred initial imaging modality for diagnosing amoebic liver abscess due to its non-invasive nature, low cost, and high sensitivity. It can detect fluid collections in the liver, which are characteristic of amoebic liver abscess. Additionally, ultrasound can guide aspiration or biopsy for further diagnostic confirmation.

**Why Each Wrong Option is Incorrect**
**Option A:** CT Scan - While CT scan can provide more detailed images of the liver, it is not the initial investigation of choice due to its higher cost and radiation exposure.
**Option B:** MRI - MRI is not typically used as an initial investigation for amoebic liver abscess due to its high cost and limited availability.
**Option C:** Blood tests (e.g., liver function tests) - Blood tests may show non-specific abnormalities in patients with amoebic liver abscess, but they are not diagnostic on their own.

**Clinical Pearl / High-Yield Fact**
The "anchovy paste" appearance on ultrasound is a characteristic feature of amoebic liver abscess, which is a result of the abscess's thick, viscous fluid.

**Correct Answer: A. Ultrasound.**