A nurse is reviewing laboratory values for a client who has systemic lupus erythematosus (SLE). Which of the following values should give the nurse the best indication of the client's renal function?

A. Blood urea nitrogen (BUN) can be influenced by factors other than kidney function, such as dehydration or protein intake, making it less specific. B. Serum sodium reflects electrolyte balance but is not a direct indicator of kidney function. C. Serum creatinine is the most reliable indicator of renal function because it reflects the glomerular filtration rate (GFR) and is less affected by other variables. D. Urine-specific gravity indicates urine concentration but does not directly measure renal function.