The best investigation to detect renal stone is-

**Core Concept**
Non-contrast computed tomography (CT) scan is the imaging modality of choice for detecting renal stones due to its high sensitivity and specificity. This is because CT scans can accurately identify the location, size, and composition of stones, including calcium, uric acid, and struvite stones.

**Why the Correct Answer is Right**
The correct answer is **non-contrast CT scan** because it provides detailed images of the kidneys, ureters, and bladder without the need for contrast agents. This is particularly useful in patients with renal impairment, allergies to contrast agents, or those who have recently undergone contrast-enhanced studies. The non-contrast CT scan can also detect stones in the ureters and bladder, which may not be visible on plain radiographs.

**Why Each Wrong Option is Incorrect**
**Option A:** **Intravenous urography (IVU)** is an older imaging modality that involves the use of contrast agents to visualize the urinary tract. While it can detect stones, it is less sensitive and specific than non-contrast CT scans, especially for smaller stones.

**Option B:** **Plain abdominal radiograph (X-ray)** is not a reliable method for detecting renal stones, especially smaller stones or those composed of uric acid or struvite. This is because many stones are radiolucent and do not appear on plain X-rays.

**Option C:** **Ultrasound** is a useful imaging modality for detecting renal stones, but it is not as sensitive as non-contrast CT scans for detecting smaller stones or those in the ureters and bladder. Ultrasound is also operator-dependent and may not provide detailed images of the urinary tract.

**Clinical Pearl / High-Yield Fact**
Remember that non-contrast CT scans are the imaging modality of choice for detecting renal stones due to their high sensitivity and specificity. This is particularly important for patients with suspected renal colic or those with a history of kidney stones.

**Correct Answer:** C. Ultrasound