What laboratory finding is commonly seen after an infection in acute post-streptococcal glomerulonephritis?

• A. Decreased Complement C3 Levels With Nephritic-Range Proteinuria and Hematuria
• B. Elevated Complement C3 Levels With Proteinuria
• C. Decreased C4 With Nephrotic-Range Proteinuria
• D. Normal Complement With Microscopic Hematuria

Answer: (A)

After a streptococcal infection, acute post-streptococcal glomerulonephritis is driven by immune complex–mediated injury in the glomeruli. Those immune complexes activate the complement system and consume C3, so the typical lab finding is a low C3 level. C4 is usually normal in this setting. Clinically this matches a nephritic picture: hematuria (often with red cell casts), and nephritic-range proteinuria, along with possible hypertension and mild oliguria. The C3 level tends to rebound to normal as the illness resolves over weeks. Therefore, the best answer is a decreased C3 level with nephritic-range proteinuria and hematuria.