Answer:
Regurgitation of acidic stomach contents into the esophagus, commonly called "heartburn"
Explanation: Esophagus is the part of the alimentary canal,which is a tube that transports food and water substance from the mouth to the stomach. Esophagus has an upper Esophagus sphincter and and the lower Esophagus sphincter. The lower Esophagus sphincter contains muscles that connects the Esophagus and the stomach. When the lower Esophagus sphincter is closed it prevents the movement of Acids and contents in the stomach from traveling backwards from the stomach. When it fails to effectively constrict it causes what is called "HEARTBURN".
