In a patient with post-streptococcal glomerulonephritis (PSGN), what laboratory finding is expected?

In post-streptococcal glomerulonephritis (PSGN), a common complication following a group A beta-hemolytic streptococcal infection, decreased levels of complement component C3 are a hallmark laboratory finding. This decrease occurs due to the consumption of complement during the inflammatory process associated with PSGN.

In PSGN, antibodies against the streptococcal antigens form immune complexes, which lead to the activation of the complement system. Specifically, C3 is consumed as part of the inflammatory response, resulting in lower serum levels. The reduction of C3 levels can persist for several weeks, even after the clinical symptoms resolve.

In contrast, other complement components, such as C4, typically remain normal in PSGN. This unique complement pattern of decreased C3, while C4 remains unchanged, helps distinguish PSGN from other renal conditions, such as systemic lupus erythematosus (SLE), where both C3 and C4 would be low.

In summary, decreased C3 levels are a direct consequence of complement activation and consumption related to the immune response triggered by streptococcal infection, making this finding characteristic of PSGN.