Which imaging modality best evaluates hydronephrosis due to obstruction?

• A. CT Urography
• B. Ultrasound
• C. MRI
• D. Plain X-ray

Answer: (B)

The main idea here is that hydronephrosis from obstruction is best seen with ultrasound because it directly shows the dilatation of the renal collecting system in a safe, quick, and widely available way. A renal ultrasound can detect expansion of the renal pelvis and calyces, assess kidney size and cortical thickness, and help distinguish obstructive hydronephrosis from other conditions that can mimic it. It also allows evaluation of the bladder and post-void residual, which can provide clues about outlet or functional obstruction, and Doppler use can offer additional information about urine flow. While CT urography gives detailed anatomy and precise obstruction localization, it uses radiation and contrast, making it less ideal as the first test. MRI offers excellent detail but is more resource-intensive and not as readily accessible in urgent settings. Plain X-ray often misses hydronephrosis entirely. So ultrasound stands out as the most appropriate initial imaging to evaluate hydronephrosis due to obstruction.