This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Other topics and Adverse effects, Subject name: Anatomy
Which of the following does not act as a second messenger 
Select and explain. 
A. Cyclic AMP, B. Inositol triphosphate, C. Diacycglycerol, D. G protein
D: G protein
Answer: D: G protein 
Explanation: Ref-KDT 6/e p48 Cyclic AMP,IP3 and DAG a second messenger where does G proteins are first messengers.IP3 and DAG increases the release of Ca+2 that acts as a third messenger