This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nervous System, Subject name: Physiology
All are second messengers EXCEPT? 
Select and explain. 
A. cAMP, B. DAG, C. IP3, D. Guanylyl cyclase
D: Guanylyl cyclase
Answer: D: Guanylyl cyclase 
Explanation: Ans. D Guanylyl cyclaseRef: Gattong, 25"' ed. pg. 59* Guanylyl cyclase is an enzyme that catalyses the formation of cGMP.* cGMP is important for vision in both rods and cones.* Second messengers bring about short term changes by altering enzyme function, triggering exocytosis and can also lead to alteration of transcription of various genes* Examples of secondary messengers# cAMP: Produced by action of adenyl cyclase enzyme and stimulates protein kinase A which catalyses phosphorylation of proteins and altering there activity. For example cholera toxin increases cAMP levels whereas pertussis toxin inhibits it.# IP3: Inositol phosphate diffuses into endoplasmic reticulum and triggers calcium release.# DAG: Diacyglycerol works with IP3.