Final answer:
The malfunction that allows reflux in GERD is the weakening or relaxation of the gastro-esophageal sphincter, a ring-like muscle located at the stomach end of the esophagus.
Step-by-step explanation:
The malfunction that allows reflux in GERD is the weakening or relaxation of the gastro-esophageal sphincter, a ring-like muscle located at the stomach end of the esophagus. This sphincter normally remains closed to prevent the contents of the stomach from entering the esophagus. If the sphincter fails to remain completely closed, the acidic contents of the stomach can flow back into the esophagus, causing heartburn and potentially leading to the development of gastroesophageal reflux disease (GERD).