This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Biochemistry
Which inhibits adenyl cyclase enzyme? 
Select and explain. 
A. Somatostatin, B. Calcitonin, C. Epinephrine, D. Thyroxine
A: Somatostatin
Answer: A: Somatostatin 
Explanation: Somatostatin acts directly on the acid-producing parietal cells a G-protein coupled receptor which inhibits adenylate cyclase, thus effectively antagonising the stimulatory effect of histamine to reduce acid secretion.Ref: DM Vasudevan, 7th edition