## **Core Concept**
The patient presents with acute onset abdominal distention and vomiting, which in the context of a known case of ulcerative colitis, raises suspicion for a complication such as toxic megacolon. Toxic megacolon is a life-threatening condition that requires immediate diagnosis and intervention. The condition involves non-obstructive colonic dilatation greater than 6 cm, often associated with signs of systemic toxicity.

## **Why the Correct Answer is Right**
The correct answer, **plain abdominal X-ray (AXR)**, is the most appropriate next investigation because it is quick, readily available, and can demonstrate the presence of colonic dilatation, which is a hallmark of toxic megacolon. In a patient with ulcerative colitis presenting with acute abdominal distention and vomiting, an AXR can rapidly provide critical information that guides immediate management decisions, such as the need for urgent surgical intervention.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a **CT abdomen** can provide detailed images and is useful in many abdominal conditions, it is not the first-line imaging choice in this acute setting due to its longer acquisition time, higher cost, and lesser immediate availability compared to plain X-rays. CT might be considered if the diagnosis is unclear after AXR or if complications need further evaluation.
- **Option B:** **Ultrasound** is not typically used for evaluating colonic dilatation or toxic megacolon, as gas-filled bowel loops can obscure the ultrasound waves, limiting its utility in this context.
- **Option D:** **Colonoscopy** might be risky in the setting of suspected toxic megacolon due to the potential for perforation and is generally contraindicated in acute toxic megacolon.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in suspected toxic megacolon, **proctoscopy or sigmoidoscopy is generally avoided** due to the risk of perforation. A plain abdominal X-ray showing colonic dilatation >6 cm with signs of systemic toxicity (fever, tachycardia, leukocytosis) supports the diagnosis. Early recognition and surgical consultation are crucial.

## **Correct Answer:** . Plain abdominal X-ray (AXR).