**Core Concept**
Serum sickness is a type of hypersensitivity reaction that occurs after the administration of foreign serum proteins, such as antivenom or immunoglobulins. This reaction is mediated by the immune system's response to the presence of these foreign proteins, leading to the activation of complement and the release of various cytokines and chemokines.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of serum sickness, which involves the formation of antigen-antibody complexes. When foreign serum proteins are introduced into the body, they are recognized as antigens by the immune system. The immune system produces antibodies against these antigens, leading to the formation of antigen-antibody complexes. These complexes can deposit in various tissues, including the skin, kidneys, and joints, leading to inflammation and tissue damage.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because serum sickness is not primarily caused by an allergic reaction to a specific antigen, but rather by the immune system's response to foreign serum proteins.
* **Option B:** This option is incorrect because serum sickness is not a type of autoimmune disease, but rather a hypersensitivity reaction to foreign substances.
* **Option C:** This option is incorrect because serum sickness is not primarily caused by an infection, but rather by the introduction of foreign serum proteins into the body.

**Clinical Pearl / High-Yield Fact**
One important aspect of serum sickness is that it can occur after the administration of various medications, including antivenom, immunoglobulins, and certain vaccines. This is because these medications contain foreign serum proteins that can trigger an immune response in some individuals.

**Correct Answer: D. Type III hypersensitivity reaction. An immune complex-mediated hypersensitivity reaction.**