This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology and Reproduction, Subject name: Physiology
All are second messengers except 
Select and explain. 
A. Guanylyl cyclase, B. cAMP, C. IP3, D. DAG
A: Guanylyl cyclase
Answer: A: Guanylyl cyclase 
Explanation: Ans. (a) Guanylyl cyclase(Ref: Ganong, 25th ed/p.56)The second messenger systems that are activated through coupling of hormone-receptor complexes by G-protein are,#Adenylyl cyclase-cAMP system#Inositol triphosphate (IP3) - Diacylglycerol (DAG) - Calcium (Ca2+) system#Guanylyl cyclase-cGMP systemGuanylyl cyclase is the enzyme responsible for formation of nitric oxide. Nitric oxide is the second messenger involved in cell signaling