**Core Concept**
Acute cholecystitis is a gallbladder inflammation often caused by gallstones blocking the cystic duct. The best investigation should accurately diagnose or confirm this condition, ideally non-invasive, and rapidly available.

**Why the Correct Answer is Right**
In acute cholecystitis, the best investigation is ultrasound (US) of the gallbladder. Ultrasound is highly sensitive and specific for detecting gallstones, gallbladder inflammation, and fluid collections. It is a non-invasive, bedside technique that provides real-time images, allowing for quick diagnosis and guiding further management.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan of the abdomen is more sensitive for detecting complications of cholecystitis, such as gangrene or perforation, but it is not the best initial investigation.

**Option B:** MRI of the abdomen is less commonly used for acute cholecystitis due to its higher cost, longer examination time, and limited availability.

**Option C:** ERCP (Endoscopic Retrograde Cholangiopancreatography) is an invasive procedure mainly used for treating bile duct stones or strictures, not for diagnosing acute cholecystitis.

**Clinical Pearl / High-Yield Fact**
In suspected acute cholecystitis, ultrasound should be performed first due to its high sensitivity and specificity, bedside availability, and non-invasive nature.

**Correct Answer: A. Ultrasound of the gallbladder.