## **Core Concept**
The investigation of choice for a ureteric stone involves imaging techniques that can accurately visualize the urinary tract and detect stones. Ureteric stones, also known as ureterolithiasis, are a common cause of acute abdominal pain and require prompt diagnosis and treatment.

## **Why the Correct Answer is Right**
The correct answer, **Non-contrast CT (Computed Tomography)**, is the gold standard for diagnosing ureteric stones. This is because non-contrast CT scans have high sensitivity and specificity for detecting stones in the urinary tract, including the ureters. They can accurately visualize the size and location of the stone, as well as any potential complications such as hydronephrosis. The mechanism behind this involves the use of X-rays to create detailed cross-sectional images of the body, which can detect the dense calcium content of most kidney stones.

## **Why Each Wrong Option is Incorrect**
- **Option A: Ultrasound**. While ultrasound is a useful initial imaging modality for assessing hydronephrosis and can sometimes visualize larger stones, it is less sensitive than non-contrast CT for detecting smaller stones and does not provide detailed information about stone size and location.
- **Option B: X-ray**. A plain X-ray of the kidneys, ureters, and bladder (KUB) can detect some stones, particularly those that are radiopaque (e.g., calcium stones). However, it is less sensitive than non-contrast CT and cannot detect all types of stones (e.g., uric acid stones).
- **Option D: MRI**. Magnetic Resonance Imaging (MRI) is not typically used for the initial evaluation of ureteric stones. While MRI can provide detailed images of the urinary tract, it is not as readily available or cost-effective as non-contrast CT and is usually reserved for specific situations.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that non-contrast CT is the investigation of choice for ureteric stones due to its high sensitivity and specificity. Additionally, it's worth noting that the majority of ureteric stones will pass spontaneously with conservative management, but larger stones or those causing significant obstruction may require intervention.

## **Correct Answer:** . **Non-contrast CT**