Hydronephrosis is a distention of the renal calyces and pelvis with urine as a result of a urinary outflow obstruction.

Use this mnemonic 'PACT SUPER' to memorize the differential diagnoses of hydronephrosis.

Unilateral is PACT:

Pelvic-uteric obstruction (congenital or acquired)
Aberrant renal vessels
Calculi
Tumours of renal pelvis

Bilateral is SUPER:

Stenosis of the urethra
Urethral valve
Prostatic enlargement
Extensive bladder tumour
Retro-peritoneal fibrosis