<s>[INST] context: Linaclotide is a poorly absorbed 14-amino-acid peptide that binds to the guanylate cylase C receptor on the luminal surface of intestinal enterocytes, activating the cystic fibrosis transmembrane chloride conductance channel and stimulating intestinal fluid secretion. Therefore it is use in irritable bowel syndrome. Lubiprostone selectively stimulates type 2 chloride channels in epithelial cells thereby causing an efflux of chloride into the intestinal lumen. The resultant fluid secretion into the gastrointestinal lumen provides a bolus effect that softens stool, increases intestinal transit, and improves symptoms of constipation. Lubiprostone acts locally within the intestinal tract, is rapidly metabolized, and has very low systemic bioavailability. Tegaserod is a serotonin 5-HT 4 paial agonist that has high affinity for 5-HT 4 receptors but no appreciable binding to 5-HT 3 or dopamine receptors. Tegaserod was approved for the treatment of patients with chronic constipation and IBS with predominant constipation. Methylnaltrexone: opioid receptor antagonist/approved for the treatment of opioid-induced constipation in patients receiving palliative care for advanced illness who have had inadequate response to other agents. It is administered as a subcutaneous injection (0.15 mg/kg) every 2 days. [/INST] question: Which of the following is a guanylate cyclase agonist used in irritable bowel syndrome? | options: A: Lubiprostine B: Linaclotide C: Tegaserod D: Methyl naltrexone | answer: B </s>