Final answer:
GERD erosion primarily affects the surfaces of the teeth in contact with the acid refluxed from the stomach into the mouth.
Step-by-step explanation:
The erosion caused by GERD (gastroesophageal reflux disease) primarily affects the surfaces of the teeth that are in contact with the acid refluxed from the stomach into the mouth. GERD is a digestive disorder in which stomach acid flows back into the esophagus, and when it reaches the mouth, it can lead to dental erosion. The acid can affect the enamel, causing it to soften and erode over time.