American Board of Internal Medicine (ABIM) Certification Practice Exam

What laboratory finding is consistent with poststreptococcal glomerulonephritis?

1. Low C3 and normal C4 levels

2. High C4 and normal C3 levels

3. Normal C3 and normal C4 levels

4. High C3 and high C4 levels

The correct answer is: Low C3 and normal C4 levels

In poststreptococcal glomerulonephritis (PSGN), a common complication following an infection with certain strains of Streptococcus bacteria, the typical laboratory finding involves complement levels. Specifically, patients with PSGN often exhibit low serum C3 levels. This finding is consistent because the underlying immune response triggered by the streptococcal infection leads to the consumption of complement components, particularly C3, as immune complexes are formed and deposited in the kidneys, causing inflammation and damage. In contrast, C4 levels typically remain normal in PSGN, as this condition mainly affects the classical complement pathway, which involves C3 more significantly than C4. Therefore, the combination of low C3 alongside normal C4 is a classic laboratory signature indicating poststreptococcal glomerulonephritis, reflecting the underlying pathophysiology of the illness rather than a deficiency or activation of the entire complement system.