**Question:** Low complement levels seen in - a) PSGN, b) MPGN, c) Good pasture's syndrome, d) Wegner's granulomatosis, e) Infective endocarditis

**Core Concept:** Complement system is a crucial part of the immune response, consisting of a series of proteins that work together to destroy pathogens, remove cellular debris, and regulate inflammation. Complement proteins act in a cascade-like manner, with each protein converting the previous one into an active form. Low complement levels can compromise the immune response and increase susceptibility to infections and autoimmune diseases.

**Why the Correct Answer is Right:** Complement system plays a vital role in the defense against pathogens, including Streptococcus pyogenes (PSGN), C3 glomerulonephritis (MPGN), Good Pasture's syndrome, and Wegener's granulomatosis, which are all conditions characterized by renal involvement. Infective endocarditis (IE) is also associated with low complement levels due to the involvement of the complement system in the host defense against bacteria and the role of complement proteins in inflammation and tissue damage in IE.

**Why Each Wrong Option is Incorrect:**

a) Membranoproliferative glomerulonephritis (MPGN) is a renal disorder characterized by the deposition of immune complexes in the glomerular basement membrane, leading to inflammation and damage. The correct answer, C3 glomerulonephritis, is a specific type of MPGN characterized by the deposition of C3 proteins. Low complement levels can exacerbate the disease process, making the correct answer the correct choice.

b) Good Pasture's syndrome is a rare disorder characterized by the deposition of immune complexes containing anti-glomerular basement membrane antibodies. Although low complement levels can worsen the disease process, this option is incorrect because Good Pasture's syndrome itself is not associated with low complement levels.

c) Wegener's granulomatosis is a systemic vasculitis characterized by granuloma formation in the kidneys, lungs, and upper respiratory tract. Low complement levels contribute to the pathogenesis of the disease, making this option incorrect.

d) Infective endocarditis (IE) is an infection of the heart valves, causing inflammation and damage. Low complement levels can impair the host defense against bacteria, making this option correct.

e) Infective endocarditis (IE) is an infection of the heart valves, causing inflammation and damage. Low complement levels can impair the host defense against bacteria, making this option correct.

f) Membranoproliferative glomerulonephritis (MPGN) is a disorder characterized by immune complex deposition in the glomerular basement membrane. Low complement levels can worsen the disease process, making this option incorrect.

g) Good Pasture's syndrome is a rare disorder characterized by the deposition of immune complexes containing anti-glomerular basement membrane antibodies. Low complement levels can worsen the disease process, making this option incorrect.

h) Wegener's granulomatosis is a systemic vasculitis characterized by granuloma formation in the kidneys, lungs, and upper respiratory tract. Low complement levels can impair the host defense against