A most common cause of urinary obstruction in a male infant is –
"The most common cause of severe obstructive uropathy in children is posterior urethral valves".___ Nelson
Posterior urethral valves

These are symmetrical folds of urothelium extending distally from prostatic urethra to external urinary sphincter.
It most commonly lies just distal to the verumontanum or at the verumontanum.
It occurs only in males.
It behaves as flap valves so, although urine does not flow normally a urethral catheter can be passed without difficulty.
Sometimes, the valves are incomplete and the patient remains without symptoms until adolescence or adulthood.
Approximately 30% of patients experience end-stage renal disease
Vesicoureteral reflux occurs in 50% of patients.
Diagnosis is made by


Voiding cystourethrogram
Endoscopy


Both of these investigations clearly depict the site of obstruction.
The diagnosis can be established prenatally by ultrasound.
Anterior urethral valves
Posterior urethral valves
Stone
Stricture