This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
Most common cause of urinary obstruction in a male infant is: 
Select and explain. 
A. Anterior urethral valves, B. Posterior urethral valves, C. Stone, D. Stricture
B: Posterior urethral valves
Answer: B: Posterior urethral valves 
Explanation: Posterior urethral valve is the most common cause of urinary obstruction in a male infant. Urethral valves are mostly posteriorly located, anterior urethral valve are usually associated with congenital urethral diveiculum. Renal calculi and stricture are uncommon in children. Ref: Kliegman, Behrman, Jenson, Stanton (2008), Chapter 540, "Obstruction of urinary tract", In the book, "NELSON TEXTBOOK OF PEDIATRICS", Volume 2, 18th Edition, New Delhi, Pages 2235 ; O P Ghai Essential Pediatrics, 6th Edition, Page 467