**Core Concept**
The BAO/MAO ratio is a measure used to assess the functionality of the adrenal glands, particularly in the context of Zollinger-Ellison Syndrome (ZES). It represents the ratio of Basal Adrenal Output (BAO) to Maximal Adrenal Output (MAO) after stimulation, typically with a glucagon or histamine challenge. An elevated BAO/MAO ratio suggests an excessive cortisol production.

**Why the Correct Answer is Right**
In ZES, the elevated gastrin levels lead to an increased secretion of gastric acid, causing a decrease in the BAO/MAO ratio. A ratio > 4 is suggestive of a diagnosis of ZES because it indicates an abnormality in the adrenal gland's ability to respond to glucocorticoid stimulation, resulting in an inappropriately low cortisol production.

**Why Each Wrong Option is Incorrect**
* **Option A:** A ratio > 4 is not indicative of normal adrenal function, which would typically have a ratio between 2-3.
* **Option B:** A ratio < 2 is suggestive of adrenal insufficiency, not ZES.
* **Option C:** A ratio > 2 but < 4 is within the normal range and does not support a diagnosis of ZES.

**Clinical Pearl / High-Yield Fact**
The BAO/MAO ratio is an important diagnostic tool in distinguishing between ZES and other causes of hypergastrinemia, such as atrophic gastritis or gastric ulcers.

**Correct Answer:** D.