What low-molecular-weight protein is a cysteine protease inhibitor that can be used as a serum biomarker for detecting acute kidney injury?

• A. Cystatin C
• B. Creatinine
• C. Beta-2 microglobulin
• D. NGAL

Answer: (A)

Cystatin C is a small, low-molecular-weight cysteine protease inhibitor produced by all nucleated cells at a constant rate and freely filtered by the glomerulus. It is then reabsorbed and degraded by the proximal tubule, so its serum concentration reflects GFR. When acute kidney injury occurs and GFR falls, cystatin C rises, often detecting dysfunction earlier than creatinine and with less dependence on muscle mass. The other options are not cysteine protease inhibitors, or are different low-molecular-weight proteins not describing the inhibitor in question (creatinine is a muscle metabolism byproduct; beta-2 microglobulin and NGAL are other biomarkers not described as cysteine protease inhibitors).