**Core Concept**
Reflux esophagitis is a condition characterized by inflammation of the esophagus due to the backflow of stomach acid. This backflow occurs when the lower esophageal sphincter (LES) is incompetent, allowing gastric contents to flow up into the esophagus.

**Why the Correct Answer is Right**
The pH of the esophagus is normally above 4, which is slightly alkaline. However, in reflux esophagitis, the pH of the esophagus is less than 4, indicating the presence of acidic gastric contents. This acidic environment can cause damage to the esophageal mucosa, leading to inflammation and symptoms such as heartburn and dysphagia. The esophageal mucosa is not adapted to handle the acidic environment, as it is normally protected by a layer of mucus and bicarbonate.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a pH of 4 is actually the threshold for normal esophageal pH, not reflux esophagitis.
* **Option B:** This option is incorrect because a pH of 3 is actually the threshold for severe esophageal acid exposure, not reflux esophagitis.
* **Option C:** This option is incorrect because a pH of 5 is actually the threshold for mild esophageal acid exposure, not reflux esophagitis.

**Clinical Pearl / High-Yield Fact**
The Montreal definition of gastroesophageal reflux disease (GERD) is based on symptoms of heartburn and regurgitation, and can be diagnosed in patients with a pH of less than 4 for more than 5% of the total time.

**Correct Answer:** D.