Which of the following is caused by deficiency of complement C3 and C3b?
**Core Concept**
The complement system is a part of the immune system that enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promoting inflammation and complement activation. Complement C3 and C3b play crucial roles in the complement cascade, with C3b being a key opsonin that helps in the phagocytosis of pathogens.
**Why the Correct Answer is Right**
Deficiency of complement C3 and C3b leads to an increased susceptibility to infections, particularly those caused by encapsulated bacteria such as Neisseria meningitidis and Streptococcus pneumoniae. This is because C3b acts as an opsonin, marking the pathogen for destruction by phagocytic cells. Without sufficient C3b, the phagocytic activity is impaired, leading to uncontrolled bacterial growth and severe infections. The deficiency of C3 can also lead to autoimmune hemolytic anemia due to the formation of immune complexes.
**Why Each Wrong Option is Incorrect**
- **Option A:** Deficiency of C3 is not directly associated with the development of rheumatoid arthritis, although it can contribute to the formation of immune complexes that exacerbate the disease.
- **Option B:** Deficiency of C3 is not directly associated with the development of atypical hemolytic uremic syndrome (aHUS), which is primarily caused by mutations in complement regulatory proteins such as factor H.
- **Option C:** Deficiency of C3 is not directly associated with the development of Goodpasture syndrome, which is primarily caused by autoantibodies against the collagen of the basement membrane.
**Clinical Pearl / High-Yield Fact**
Deficiency of complement C3 can lead to an increased risk of infections, particularly those caused by encapsulated bacteria, and should be considered in patients with recurrent infections or unexplained hemolytic anemia.
**Correct Answer:** C.