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: Biochemistry
The cyclic AMP is inactivated by the action of which specific enzyme? 
Select and explain. 
A. Phosphodiesterase, B. Phosphate, C. Protein kinase, D. Adenyl cyclase
A: Phosphodiesterase
Answer: A: Phosphodiesterase 
Explanation: cAMP undergoes rapid hydrolysis catalysed by the enzyme phosphodiesterase, to 5'AMP which is inactive. Caffeine and theophylline can inhibit phosphodiesterase and increase the intracellular levels of cAMP.