What is the primary anatomical/physiological cause of gastroesophageal reflux disease (GERD)?

GERD occurs primarily due to the failure of the lower esophageal sphincter (LES) to maintain adequate pressure, allowing gastric acid and pepsin to reflux into the esophagus. The LES is a specialized area of smooth muscle at the gastroesophageal junction that normally maintains a higher pressure than the stomach, preventing reflux. When the LES is hypotensive, transiently relaxes, or is physically displaced (as in hiatal hernia), acid reflux occurs. In dentistry, GERD is significant because chronic acid exposure causes palatal erosion (particularly of the maxillary anterior teeth) and perimolysis. Dental erosion from GERD typically affects the palatal surfaces of maxillary teeth — a distribution that distinguishes GERD from dietary erosion.