**Core Concept**
Minimal ascites refers to a small amount of fluid accumulation in the peritoneal cavity, which can be challenging to detect using various imaging modalities. The key principle in detecting minimal ascites is to identify the presence of fluid in the abdominal cavity, which can be achieved through various imaging techniques.

**Why the Correct Answer is Right**
Ultrasound (USG) is the most sensitive and specific imaging modality for detecting minimal ascites. This is because sound waves can easily penetrate the abdominal wall and detect even small amounts of fluid in the peritoneal cavity. The fluid appears as an anechoic area (black) on the ultrasound image, making it easy to identify. Additionally, ultrasound is non-invasive, quick, and does not expose the patient to radiation, making it an ideal choice for detecting minimal ascites.

**Why Each Wrong Option is Incorrect**
**Option B:** Plain X-ray abdomen is less sensitive than ultrasound for detecting minimal ascites. This is because a small amount of fluid may not be visible on a plain X-ray, and the presence of bowel gas and other abdominal structures can obscure the view.

**Option C:** MRI is not typically used for detecting minimal ascites due to its high cost, limited availability, and longer examination time. Additionally, MRI may not be as sensitive as ultrasound for detecting small amounts of fluid in the peritoneal cavity.

**Option D:** CT scan is more sensitive than plain X-ray but less sensitive than ultrasound for detecting minimal ascites. CT scans involve radiation exposure and are more expensive than ultrasound, making them a less ideal choice for detecting small amounts of fluid in the abdominal cavity.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with suspected ascites, it is essential to perform a thorough physical examination, including abdominal palpation, to detect any signs of fluid accumulation. Minimal ascites may not be visible on clinical examination, but ultrasound can help detect even small amounts of fluid in the peritoneal cavity.

**✓ Correct Answer: A. USG**