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
All are second messengers except: 
Select and explain. 
A. Cyclic AMP, B. Guanylyl cyclase, C. Diacylglycerol, D. Inositol triphosphate
B: Guanylyl cyclase
Answer: B: Guanylyl cyclase 
Explanation: Remember that guanylyl cyclase is an enzyme while cGMP is a secondary messenger
Types of second messengers
There are 3 basic types of secondary messenger molecules:
1. Hydrophobic

DAG
IP3
Phosphotidylinositol

2. Hydrophilic

cAMP
cGMP

3. Gases

NO
CO