If plasma glucose reaches about 200 mg/dL, which statement is most accurate?

• A. Glucose remains entirely in the blood with no urinary excretion
• B. Glucose is excreted in urine only if the liver cannot metabolize it
• C. Glucose is excreted in urine regardless of plasma levels
• D. Glucose begins to appear in urine because plasma levels exceed renal threshold

Answer: (D)

The key idea is how the kidney handles glucose. Glucose is filtered into the tubules, but the proximal tubule reabsorbs almost all of it via transporters until a maximum rate is reached. That set point is the renal threshold for glucose, around 180 mg/dL. When plasma glucose rises to about 200 mg/dL, the filtered load of glucose exceeds the reabsorptive capacity (the transport maximum), so not all glucose can be reabsorbed. The excess spills into the urine, causing glucosuria. This happens regardless of what the liver is doing with glucose, because the limiting factor here is the kidney’s reabsorptive capacity, not hepatic metabolism.

So, glucose begins to appear in urine because plasma levels exceed renal threshold.