The obstruction may be either partial or complete, and can occur anywhere from the urethral meatus to the renal calyces. Hydronephrosis can also result from the retrograde flow of urine from the bladder back into the kidneys (vesicoureteral reflux), which can be caused by some of the factors listed above as well as compression of the bladder outlet into the urethra by prostate enlargement or fecal impaction in the rectum (which sits immediately behind the prostate), as well as abnormal contractions of bladder detrusor muscles resulting from neurological dysfunction (neurogenic bladder) or other muscular disorders.