A 6-year-old boy undergoes abdominal ultrasound for recurrent urinary tract infections. The imaging shows dilated renal pelvis and calyces with the finding marked **C** (reduced renal cortex thickness). According to the Society of Fetal Urology (SFU) grading system, which grade of hydronephrosis is most consistent with this sonographic appearance?

Explanation

## Why Grade 4 is right The presence of reduced renal cortex thickness (marked **C**) in the setting of dilated renal pelvis and calyces is the defining sonographic feature of SFU Grade 4 hydronephrosis. According to Campbell-Walsh Urology and the Society of Fetal Urology classification, Grade 4 specifically includes parenchymal thinning (cortical atrophy) combined with dilation of both major and minor calyces. This indicates chronic obstruction with progressive loss of renal function and is the most severe grade of hydronephrosis on the SFU scale. ## Why each distractor is wrong - **Grade 1**: Characterized only by mild splitting of the renal sinus echo with no visualization of calyces or parenchymal changes. The presence of reduced cortex thickness rules out this mild grade. - **Grade 2**: Shows splitting of the renal sinus and visualization of major calyces only, but does not include parenchymal thinning. The cortical atrophy seen here indicates more advanced disease. - **Grade 3**: Includes visualization of both major and minor calyces but explicitly excludes parenchymal thinning. The reduced cortex thickness (**C**) is the key finding that distinguishes Grade 4 from Grade 3. **High-Yield:** SFU Grade 4 = parenchymal thinning + calyceal dilation; this indicates chronic obstruction with renal cortical loss and warrants urgent evaluation for the underlying cause (UPJ obstruction, VUR, duplex system) and assessment of differential renal function. [cite: Campbell-Walsh Urology 12e; Society of Fetal Urology Grading System for Hydronephrosis]