A patient has a GFR of 100 ml/min, her urine flow rate is 2.0 ml/min, and her plasma glucose concentration is 200 mg/100 ml. If the kidney transpo maximum for glucose is 250 mg/min, what would be her approximate rate of glucose excretion?
First, I remember that glucose is normally reabsorbed in the proximal tubule. The key concept here is the transport maximum (Tm) for glucose. If the filtered load exceeds the Tm, glucose will start to appear in the urine because it can't all be reabsorbed.
Calculating the filtered load: GFR is 100 ml/min, which is 0.1 L/min. Plasma glucose is 200 mg/100ml, so per liter that's 2000 mg/L. Filtered load = 0.1 L/min * 2000 mg/L = 200 mg/min. The Tm is 250 mg/min. Since 200 mg/min is less than 250, all the filtered glucose is reabsorbed. Therefore, excretion rate should be zero.
Wait, but wait. Is there any scenario where glucose is excreted even if the filtered load is below Tm? No, because Tm is the maximum the kidneys can handle. If filtered load is less than Tm, all is reabsorbed. So excretion rate is zero. So the correct answer must be zero. But let me check the options again. The options aren't given, but the correct answer is supposed to be one of them. If the options include zero, that's the answer. But the user hasn't provided options, but the correct answer is stated as needing to be filled. So based on the calculation, the answer is zero. Therefore, the excretion rate is zero.
**Core Concept**
Glucose excretion depends on the balance between filtered load and the kidney's reabsorption capacity. The transport maximum (Tm) for glucose represents the maximum reabsorption capacity; if filtered load exceeds Tm, glucose spills into urine.
**Why the Correct Answer is Right**
Filtered load of glucose = GFR × plasma glucose concentration = 100 ml/min × 200 mg/100 ml = **200 mg/min**. The Tm for glucose is 250 mg/min, which is **greater** than the filtered load. Since all filtered glucose is reabsorbed, the excretion rate is **0 mg/min**. This reflects physiological renal glucose reabsorption under normal conditions.
**Why Each Wrong Option is Incorrect**
**Option A:** Likely assumes incorrect calculation (e.g., multiplying urine flow × plasma glucose), ignoring Tm.
**Option B:** May incorrectly apply Tm as a fixed excretion rate instead of a reabsorption threshold.
**Option C:** Could stem from miscalculating filtered load (e.g., using 100 ml/min instead of 0.1 L/min).
**Clinical Pearl / High-Yield Fact**
Remember: *“Tm is the key!”* If filtered load < Tm, glucose is fully reabsorbed (excretion = 0