## Core Concept
The investigation of choice for acute cholecystitis involves imaging techniques that can accurately diagnose inflammation of the gallbladder. Acute cholecystitis is a condition characterized by inflammation of the gallbladder, often caused by gallstones obstructing the cystic duct. The diagnostic approach aims to confirm the presence of inflammation and potential complications.

## Why the Correct Answer is Right
The correct answer, **Ultrasonography (USG)**, is the investigation of choice for acute cholecystitis because it is highly sensitive and specific for detecting gallstones, gallbladder inflammation, and signs of cholecystitis such as the presence of pericholecystic fluid, wall thickening, and the sonographic Murphy's sign. USG is non-invasive, widely available, and does not expose patients to radiation, making it an ideal initial imaging modality.

## Why Each Wrong Option is Incorrect
- **Option A: CT Scan** - While a CT scan can provide detailed images and is useful for detecting complications of acute cholecystitis, it is not the first-line imaging modality due to higher costs, radiation exposure, and lower sensitivity for gallstones compared to USG.
- **Option B: MRI** - MRI can be used for evaluating the biliary system, especially with MRCP (Magnetic Resonance Cholangiopancreatography), but it is not the initial investigation of choice for acute cholecystitis due to its higher cost, limited availability, and the fact that it provides little additional benefit over USG for initial diagnosis.
- **Option D: Endoscopic Retrograde Cholangiopancreatography (ERCP)** - ERCP is an invasive procedure primarily used for therapeutic interventions, such as stone extraction or stenting, rather than for the initial diagnosis of acute cholecystitis.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **sonographic Murphy's sign**, which is tenderness upon compression of the gallbladder with the ultrasound probe during inspiration, is a specific finding for acute cholecystitis. USG also allows for the assessment of the gallbladder and biliary ducts in a single examination, making it highly valuable in the acute setting.

## Correct Answer: . Ultrasonography (USG)