## **Core Concept**
The patient's symptoms of abdominal pain, distension, and absolute constipation suggest a condition known as **intestinal obstruction**. This condition requires prompt diagnosis and management to prevent complications. The investigation of choice for diagnosing intestinal obstruction involves imaging studies.

## **Why the Correct Answer is Right**
The correct answer, **. (X-ray abdomen)**, is the investigation of choice because an abdominal X-ray is quick, readily available, and highly sensitive for diagnosing intestinal obstruction. It can show dilated bowel loops, air-fluid levels, and sometimes the site of obstruction. This information is crucial for initial assessment and decision-making.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While ultrasound can be useful in some abdominal conditions, it is not the first-line imaging modality for suspected intestinal obstruction due to its limitations in visualizing the entire bowel and detecting air-fluid levels.
- **Option B:** CT abdomen is a more detailed imaging modality that can help identify the cause of obstruction (e.g., tumors, hernias) but is not the initial investigation of choice due to its higher cost, radiation exposure, and longer acquisition time compared to X-ray.
- **Option C:** MRI is not typically used in the acute setting for intestinal obstruction due to its long acquisition time, limited availability, and lesser utility in detecting the signs of obstruction compared to X-ray or CT.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in a patient with suspected intestinal obstruction, a plain **erect abdominal X-ray** is often the first imaging done. It can show both dilated bowel loops and air-fluid levels, which are indicative of obstruction. Remember, the presence of **pneumatosis intestinalis** (air in the bowel wall) or **free air under the diaphragm** (indicative of perforation) would significantly alter management.

## **Correct Answer:** . X-ray abdomen