Serum C3 is persistently low in the following except?
**Core Concept**
Serum C3 is a crucial component of the complement system, playing a vital role in the immune response. Persistently low levels of C3 suggest a dysregulation in the complement cascade, often associated with autoimmune or inflammatory disorders.
**Why the Correct Answer is Right**
The complement system is a complex network of proteins that help eliminate pathogens from the body. C3 is a central component that is cleaved into C3a and C3b, which then trigger a cascade of reactions leading to the destruction of pathogens. Persistently low levels of C3 can be seen in conditions like systemic lupus erythematosus (SLE), where the immune system produces autoantibodies that activate the complement system, leading to C3 consumption. This results in a deficiency of C3, which can be a diagnostic clue for SLE.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because SLE is a classic condition associated with low C3 levels due to complement activation.
**Option B:** Incorrect because membranoproliferative glomerulonephritis (MPGN) is a kidney disorder characterized by immune complex deposition, which can lead to C3 consumption and low levels.
**Option C:** Incorrect because post-streptococcal glomerulonephritis (PSGN) is an immune-mediated condition that can cause low C3 levels due to complement activation.
**Clinical Pearl / High-Yield Fact**
In SLE, low C3 levels are often associated with renal involvement and a higher risk of thrombotic complications.
**Correct Answer:** D.