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 known effects of cyclic AMP in eukaryotic cells results from: 
Select and explain. 
A. Activation of the catalytic unit of adenylate cyclase, B. Activation of synthetase, C. Activation of protein kinase, D. Phosphorylation of G protein
C: Activation of protein kinase
Answer: C: Activation of protein kinase 
Explanation: Cyclic AMP exerts most of its effects by stimulating cAMP-dependent protein kinases. These phosphorylate enzymes resulting in their activation or inhibition.