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: Pharmacology
Sildenafil's mechanism of action can be best described as: 
Select and explain. 
A. β–adrenoceptor blocking agent, B. Selective inhibitor of phosphodiesterase type 5, C. Inhibits reuptake of both serotonin and nor adrenaline, D. Selective serotonin reuptake inhibitor
B: Selective inhibitor of phosphodiesterase type 5
Answer: B: Selective inhibitor of phosphodiesterase type 5 
Explanation: None