Final answer:
GERD is caused by an incompetent gastric valve, which fails to prevent the backflow of stomach contents into the esophagus, potentially causing damage.
Step-by-step explanation:
The essential problem that causes Gastroesophageal Reflux Disease (GERD) is A. an incompetent gastric valve, also known as the gastro-esophageal sphincter or lower esophageal sphincter. This sphincter separates the esophagus from the stomach and normally remains closed to prevent stomach contents from entering the esophagus. When the sphincter fails to remain completely closed, it allows highly acidic stomach contents to travel backward into the esophagus, which can lead to heartburn and, if it occurs chronically, to GERD, potentially damaging the mucous membrane of the esophagus.