**Core Concept**
Eosinopenia is a condition characterized by a decreased level of eosinophils in the blood. It is often a response to stress, glucocorticoids, or certain medications. Eosinophils play a crucial role in the body's immune response, particularly in fighting parasites and in inflammatory reactions.

**Why the Correct Answer is Right**
The correct answer is related to a condition that causes a decrease in eosinophils due to the effect of glucocorticoids. Glucocorticoids are a class of steroid hormones that have anti-inflammatory and immunosuppressive properties. They are produced by the adrenal gland and can also be administered as medications. When glucocorticoids bind to their receptors, they trigger a cascade of reactions that ultimately lead to the suppression of eosinophil production in the bone marrow. This results in a decrease in the number of eosinophils in the blood, causing eosinopenia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly relate to a condition that causes a decrease in eosinophils due to glucocorticoids.
**Option B:** This option is incorrect because it is a condition that is associated with an increase in eosinophils, not a decrease.
**Option C:** This option is incorrect because it does not directly relate to a condition that causes a decrease in eosinophils due to glucocorticoids.

**Clinical Pearl / High-Yield Fact**
One important point to remember is that eosinopenia can be a useful indicator of the body's response to stress or glucocorticoid therapy. It can also be a sign of underlying conditions such as Cushing's syndrome or Addison's disease.

**Correct Answer:** C. Cushing's Syndrome