**Core Concept:** Obstructive jaundice is a clinical syndrome characterized by obstructed bile flow leading to elevated serum bilirubin levels, pruritus, and cholestasis. It can be caused by various obstructions in the biliary tree, such as gallstones, tumors, or post-surgical strictures. The clinical presentation often includes right upper quadrant pain radiating to the right shoulder and worsening with food intake, especially fatty foods.

**Why the Correct Answer is Right:** The correct answer is **Option C: Ultrasonography (USG)** because it is a non-invasive, readily available, and cost-effective imaging modality. Ultrasonography is the initial step in assessing patients with obstructive jaundice, as it can demonstrate gallstones, tumors, or other obstructions in the biliary tree.

**Why Each Wrong Option is Inaccurate:**

A. **Option A: CT scan:** Although CT scan is a more specific modality to visualize biliary obstructions, it is not the first-line investigation for obstructive jaundice, as it is more expensive, requires ionizing radiation exposure, and may not be readily available in all settings.

B. **Option B: MRI:** MRI is not the first choice for evaluating obstructive jaundice due to its limited availability, higher cost, and longer imaging time compared to ultrasonography.

D. **Option D: Endoscopic retrograde cholangiopancreatography (ERCP):** ERCP is an invasive procedure involving the insertion of a flexible tube into the digestive tract via the mouth and into the bile ducts. Although it can visualize the biliary tree, it is an invasive procedure and not the first-line investigation for obstructive jaundice due to its invasiveness, risk of complications, and limited availability.

**Clinical Pearl:** In cases where ultrasonography is inconclusive, further investigations like **CT scan** or **MRCP (Magnetic Resonance Cholangiopancreatography)** can be done to confirm the diagnosis and plan management.

**Explanation:**

The correct answer, ultrasound (USG), is chosen due to its non-invasive nature, wide availability, and capability to visualize the bile ducts and gallbladder. It enables the detection of gallstones, liver lesions, and biliary tract dilatation, helping in the diagnosis of obstructive jaundice.

In contrast, the wrong answers are incorrect for the following reasons:

1. Computed tomography (CT) scan is more specific but invasive, requires ionizing radiation exposure, and may not be accessible in all settings.
2. Magnetic resonance cholangiopancreatography (MRCP) is an invasive procedure, has limited availability in some centres, and is more expensive than ultrasound.
3. Endoscopic retrograde cholangiopancreatography (ERCP) is an invasive procedure with potential complications and limited availability.