**Core Concept**
The anti-inflammatory action of corticosteroids is primarily mediated through their ability to influence gene expression, particularly by inhibiting the production of pro-inflammatory mediators. This is achieved by acting on specific molecular pathways. Corticosteroids are **glucocorticoids**, a class of **steroid hormones** that play a crucial role in regulating the immune response and reducing inflammation.

**Why the Correct Answer is Right**
The correct answer involves the inhibition of **phospholipase A2**, an enzyme crucial for the production of **arachidonic acid**, which is further metabolized into **prostaglandins** and **leukotrienes**, key pro-inflammatory mediators. By inducing the production of **lipocortin**, which inhibits **phospholipase A2**, corticosteroids effectively block the synthesis of these inflammatory substances, thereby exerting their anti-inflammatory effects.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while corticosteroids do have immunosuppressive effects, their primary anti-inflammatory action is not through direct immunosuppression but through inhibition of specific inflammatory pathways.
**Option B:** Incorrect as it is not directly related to the primary mechanism of anti-inflammatory action of corticosteroids.
**Option C:** Incorrect because, although corticosteroids can influence various aspects of the inflammatory response, this option does not accurately describe the primary mechanism of their anti-inflammatory action.
**Option D:** Incorrect as it is not the primary mechanism by which corticosteroids exert their anti-inflammatory effects.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that corticosteroids, due to their broad mechanism of action, can suppress a wide range of inflammatory processes, making them useful in treating various conditions, from asthma to rheumatoid arthritis. However, their use must be carefully considered due to potential side effects.

**Correct Answer:** D. phospholipase A2