patient has blood glucose of 200 mg / dl, GFR = 90 ml / min, transport maximum for glucose = 150 mg / min. What is the amount of glucose excreted?
**Core Concept**
The glucose transport maximum (Tm) is the maximum rate at which the kidneys can reabsorb glucose from the filtrate back into the bloodstream. The kidneys reabsorb glucose using a sodium-glucose cotransporter (SGLT2), which is located in the proximal convoluted tubule. When the filtered glucose load exceeds the Tm, glucose begins to appear in the urine.
**Why the Correct Answer is Right**
Given a blood glucose of 200 mg/dL and a GFR of 180 mL/min (assuming 90 mL/min is the GFR for one kidney), the filtered glucose load is calculated as follows:
Filtered glucose load = GFR × Blood glucose concentration
Filtered glucose load = 180 mL/min × 200 mg/dL
Filtered glucose load = 36000 mg/min
However, the transport maximum for glucose (Tm) is 150 mg/min. Since the filtered glucose load exceeds the Tm, glucose begins to appear in the urine. The amount of glucose excreted can be calculated by subtracting the Tm from the filtered glucose load:
Amount of glucose excreted = Filtered glucose load - Tm
Amount of glucose excreted = 36000 mg/min - 150 mg/min
Amount of glucose excreted = 35850 mg/min
**Why Each Wrong Option is Incorrect**
**Option A:** This option does not account for the glucose load that exceeds the Tm, which results in glucose appearing in the urine.
**Option B:** This option assumes that all filtered glucose is reabsorbed, which is not the case when the glucose load exceeds the Tm.
**Option C:** This option incorrectly calculates the amount of glucose excreted by subtracting the GFR from the Tm, rather than subtracting the Tm from the filtered glucose load.
**Clinical Pearl / High-Yield Fact**
When the glucose load exceeds the Tm, glucose begins to appear in the urine, leading to glycosuria. This is an important clinical finding that can indicate impaired glucose regulation or renal dysfunction.
**Correct Answer:** D. 35850 mg/min