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
Linaclotide is a 
Select and explain. 
A. Chloride channel activator, B. CFTR blocker, C. Somatostatin antagonist, D. Guanylate cyclase agonist
D: Guanylate cyclase agonist
Answer: D: Guanylate cyclase agonist 
Explanation: Linaclotide is a Guanylate cyclase - C agonist that increases levels of cGMP and stimulates secretion of chloride & bicarbonate ions into the intestinal lumen. It is used in treatment of IBS with constipation.