What is one of the most common symptoms associated with GERD?

Heartburn is one of the hallmark symptoms of gastroesophageal reflux disease (GERD). It occurs when stomach acid flows back into the esophagus, causing a burning sensation just behind the breastbone. This discomfort typically worsens after meals or when lying down and can be accompanied by a bitter or sour taste in the mouth due to the reflux of stomach contents.

The presence of heartburn is significant in diagnosing GERD because it directly relates to the underlying pathophysiology of the condition, which involves a malfunctioning lower esophageal sphincter. While other gastrointestinal symptoms, such as nausea, may occur and are associated with various other conditions, they are not as characteristic of GERD as heartburn. Diarrhea and constipation are typical symptoms associated with different gastrointestinal disorders and would not indicate GERD. Thus, the prominence of heartburn in the symptomatology of GERD makes it the correct answer in this context.