For suspected renal colic, which imaging modality is preferred, and which findings support stone disease?

• A. Ultrasound is the preferred imaging modality; hydronephrosis is not informative.
• B. MRI is the preferred modality; it shows stones clearly.
• C. Noncontrast CT is preferred; findings include a visible stone and/or hydronephrosis.
• D. Plain abdominal X-ray is preferred; absence of edema rules out stone.

Answer: (C)

When renal colic is suspected, the priority is to quickly confirm a stone and assess whether there’s an obstruction. Noncontrast CT of the abdomen/pelvis is the imaging modality of choice because it can directly visualize stones of all compositions, pinpoint their exact location and size, and identify signs of obstruction such as hydronephrosis. Hydronephrosis—dilation of the renal pelvis and calyces upstream of an obstruction—supports the diagnosis of stone-related blockage. CT is fast, widely available, and does not require contrast, making it superior for this scenario.

Ultrasound can detect hydronephrosis and is preferred in pregnancy or in children, but it is less sensitive for detecting ureteral stones, especially small ones. MRI is not effective for visualizing stones. Plain abdominal X-ray can miss many stones (especially radiolucent ones) and does not reliably show obstruction.