## **Core Concept**
The investigation of choice for peptic ulcer perforation involves identifying the most accurate and rapid diagnostic method for detecting free air under the diaphragm or within the abdominal cavity, which is a hallmark of perforation. This condition is a medical emergency requiring immediate diagnosis and treatment.

## **Why the Correct Answer is Right**
The correct answer, **. (Abdominal X-ray)**, is right because an erect chest X-ray or an erect abdominal X-ray is often the first imaging done in suspected cases of peptic ulcer perforation. It can show free air under the diaphragm in up to 70-80% of cases, which is indicative of a perforated viscus. This method is quick, readily available, and can be performed in an unstable patient.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While ultrasound can be useful in many abdominal conditions, it is not the first-line investigation for suspected peptic ulcer perforation because it may not detect free air as effectively as an X-ray and is operator-dependent.
- **Option B:** CT abdomen with contrast is highly sensitive and specific for diagnosing peptic ulcer perforation and can identify the site of perforation. However, it is not typically the first-line investigation due to its higher cost, availability, and the time required to perform it, especially in unstable patients.
- **Option C:** MRI is not routinely used in the acute setting of suspected peptic ulcer perforation due to its limited availability, higher cost, and the time it takes to perform the scan.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in cases of suspected peptic ulcer perforation, keeping the patient in an upright position for the X-ray (erect chest or abdominal X-ray) increases the sensitivity of detecting free air under the diaphragm. This simple maneuver can significantly aid in the rapid diagnosis of this life-threatening condition.

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