**Core Concept**
The early effect of primary trauma from occlusion refers to the immediate response of the dental pulp to the sudden loss of blood flow and oxygenation caused by tooth occlusion. This response involves the activation of various cellular and molecular pathways that lead to the initiation of an inflammatory response.

**Why the Correct Answer is Right**
The correct answer is associated with the release of pro-inflammatory mediators, such as bradykinin, prostaglandins, and histamine, which are produced by the pulp cells in response to the trauma. These mediators cause vasodilation, increased blood flow, and permeability of the pulp vessels, leading to the influx of inflammatory cells and the release of pain-producing substances. This early response is mediated by the activation of phospholipase A2 (PLA2) and the subsequent production of arachidonic acid metabolites.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the early effect of primary trauma from occlusion. The correct answer is associated with the release of pro-inflammatory mediators, not the activation of an immune response.
**Option B:** This option is incorrect because it is related to the late effect of primary trauma, not the early effect. The late effect involves the activation of immune cells and the release of cytokines, which occurs hours or days after the initial trauma.
**Option C:** This option is incorrect because it is related to the effect of secondary trauma, not primary trauma. Secondary trauma refers to the damage caused by the body's response to the initial trauma, rather than the initial trauma itself.
**Option D:** This option is incorrect because it is related to the effect of bacterial infection, not primary trauma from occlusion.

**Clinical Pearl / High-Yield Fact**
The early effect of primary trauma from occlusion is a critical period for the initiation of an inflammatory response, and the release of pro-inflammatory mediators is a key event in this process. Understanding this response is essential for the diagnosis and treatment of dental pain and pulpitis.

**Correct Answer:** A. Pain and inflammation.