## **Core Concept**
Immune complex diseases occur when antibodies bind to soluble antigens, forming immune complexes that can deposit in tissues, leading to inflammation and tissue damage. This process involves the complement system and various immune cells. The diseases are characterized by the presence of immune complexes and the resultant inflammatory response.

## **Why the Correct Answer is Right**
Post-streptococcal glomerulonephritis (PSGN) is an example of an immune complex disease. It occurs after a streptococcal infection, where antibodies against streptococcal antigens cross-react with glomerular antigens, forming immune complexes that deposit in the kidneys, leading to inflammation. This condition is a classic example of an immune complex-mediated disease.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Serum sickness is an immune complex disease caused by the administration of antiserum or certain drugs, leading to the formation of immune complexes that deposit in tissues.
* **Option B:** Systemic lupus erythematosus (SLE) is a prototypic immune complex disease, characterized by the deposition of immune complexes in various tissues, including the skin, kidneys, and joints.
* **Option D:** Rheumatoid arthritis (RA) involves immune complex formation and deposition in the synovium, contributing to joint inflammation and damage.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that immune complex diseases often involve the complement system, and low levels of complement (C3 and C4) can be a marker of active disease. For example, in PSGN, C3 levels are typically low during the acute phase but normalize within 8-12 weeks.

## **Correct Answer:** . Post-streptococcal glomerulonephritis