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: Radiology
Key hole sign is seen in 
Select and explain. 
A. Hydronephrosis, B. Posterior urethral valve, C. Ectopic ureter, D. PCKD
B: Posterior urethral valve
Answer: B: Posterior urethral valve 
Explanation: Key hole sign - Posterior urethral valve.
Crescent sign - Hydronephrosis.