The GFR of a 26-year-old man with glomerulonephritis decreases by 50% and remains at that level. For which substance would you expect to find the greatest increase in plasma concentration?

A 50% reduction of GFR would approximately double the plasma creatinine concentration because creatinine is not reabsorbed or secreted and its excretion depends largely on glomerular filtration. Therefore, when GFR decreases, the plasma concentration of creatinine increases until the renal excretion of creatinine returns to normal. Plasma concentrations of glucose, potassium, sodium, and hydrogen ions are closely regulated by multiple mechanisms that keep them relatively constant even when GFR falls to very low levels.