Which indirect marker of kidney function (GFR) is less influenced by muscle mass?

• A. Creatinine
• B. Cystatin C
• C. Urea
• D. Albumin

Answer: (B)

Estimating how well the kidneys filter blood relies on markers that reflect filtration rather than production. Creatinine, a byproduct of muscle metabolism, varies with muscle mass and age, so people with more muscle can have higher creatinine even if their GFR is normal, while those with less muscle may seem to have better filtration than they actually do. Cystatin C is produced at a fairly constant rate by all nucleated cells and is freely filtered by the glomerulus; its level in the blood rises when GFR falls but is less affected by how much muscle a person has. Urea (BUN) changes with protein intake, hydration, liver function, and catabolic state, which can mislead about true filtration. Albumin is driven by nutritional status and inflammation, not by kidney filtration. So cystatin C serves as a less muscle-mass–dependent marker of kidney function, making it the best choice in this context.