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#1




What would be the rate of urinary glucose excretion?
A 32 y/o man complains of frequent urination. He is overweight (280 pounds, 5 feet 10 inches tall), and after measuring the 24 h creatinine clearance, you estimate his GFR to be 150 ml/min. His plasma glucose is 300 mg/dl. Assuming that his renal transport maximum for glucose is normal, as shown in the figure below, what would be this patient's approximate rate of urinary glucose excretion?
qs.bmp A) 0 mg/min B) 100 mg/min C) 150 mg/min D) 225 mg/min E) 300 mg/min F) Not enough information is given to estimate the glucose excretion rate. xaxis should read 100,200 & 300 Last edited by bebix; 07052011 at 01:57 PM. Reason: xaxis should read 100,200 & 300 
#4




Sorry, I was confused by the graph, I rather choose B. 100mg/min. The graph has two 300, the first should be read as 200.

#5




yup..the threshold is 225. thx

#7




Oops my pc could not open the graph so i was gone wrong..since i cannot open the graph can u plz tell me how u calculated? As per what i understood if his transport maximum is normal then his glucose excretion should be zero..but if excretion is 1oo then he should be diabetic. Am i right? Please correct me if i am wrong. Thnks in advance

#8




Quote:
click image to enlarge 
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Ace3 (07052011) 
#10




After Carefully review the question, i got the answer, i got it wrong the first time...
It should be something like this... FL = GFR x [ Plasma ] So Doing this, is like FL = 150 x [ 300 mg / 300 ml ]... FL = 150 <> Hence Answer C. Thanks Bebix... =) Last edited by rulz; 07052011 at 04:31 PM. Reason: Review the post... 
#12




i was saying it is d but when i say that transport system is normal i am really confused about the answer. i know it should appear in urine after tm is saturated

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bebix (07052011) 
#13




D?
blood glucose is 300 mg/dl = 300 mg/100 ml. With GFR 150 ml/min there would be 450 mg glucose passing through in a minute, the threshold is 225 (from graph), so other 450225=225 would be excreted....

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docji (07062011) 
#16




(C)
(C)
Well my rationale is as follows : The patients weight and height were given ,, If we used them to get the body surface area according to The Mosteller¹ formula , it would be as follows : BSA (m²) = ( [ Height(in) x Weight(lbs) ]/ 3131 )½ So BSA (m²) = ( [ 60 (in) x 280 (lbs) ]/ 3131 )½ = 2.3 m² (I guess that the estimated GFR given above was for the standard surface area which is usually 1.73 m² so the given GFR is 150ml / 1.73 m² / min) So the correction of the GFR would be (2.3*150)/1.73 = 200ml/min Now the threshold is 225 meaning any value above will be excreted so the excreted amount if the concentration was 300mg glucose / dl will be as follows 300225 = 75 mg / dl / min and since 2 dl of blood are filtered in a min So 150 mg of glucose will be excreted in a min so the right answer would be (C) 
#17




correct answer
The correct answer is C)
GFR = 150 ml/min Glucose = 300 mg/dl => 1 dL = 100 ml => 300mg/100ml Filtered Load = GFR * Glucose => 150 ml/min * 300mg/100ml = 450 mg/min Tmax = 300 mg/min Then, 450 mg/min  300 mg/min = 150 mg/min 
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#18




D
Threshold is the point where glucose starts appearing in the urine.
Saturation is the point from where a constant amount of glucose will be reabsorbed (i.e. the Tmax) and anything beyond that will be excreted. SO in this question........glucose will start appearing in the urine at threshold value.......that means if 450mg/min is filtered load.......then out of that 225 will be reabsorbed.........and rest will be excreted...... and as above calculated correctly answer shoulde be D according to me. please share your views and tell me if I am wrong in this concept. 
#19




Quote:
When the filtered load is 225 mg/min, a small amount of glucose begins to appear in the urine (threshold for glucose). This appearance of glucose in the urine (at the threshold) occurs before the transport maximum is reached. One reason for the difference between threshold and transport maximum is that not all nephrons have the same transport maximum for glucose, and some of the nephrons excrete glucose before others have reached their transport maximum. The overall transport maximum for the kidneys, which in this case is 300 mg/min, is reached when all nephrons have reached their maximal capacity to reabsorb glucose. Since the question is what would be this patient's approximate rate of urinary glucose excretion?...225 mg/min is not the BEST answer, because we know that NOT all have reached their maximal capacity...the best answer in this case is 150 mg/min 
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#20




Cannot be anything but d
HAS TO BE D.
Because after threshold is when you see glucose start to appearing in urine first. Why are you guys excluding that. IT has to be 450  225 = 225. If you are doing 450  300 (tm) = 150 mg/ml. I mean even the Qbank says its right. IT HAS TO BE WRONG. Because what you are saying is that you dont see glucose in urine until you reach 300mg/ml (tm). But in the contrary, you start seeing glucose in urine as soon as you pass threshold. The question is asking what is the approximate rate of urinary glucose excretion, so because urine is excreted right after you pass the threshold you definitely shouldnt you use 225 in the equation rather than 300???? 
#21




Quote:
the approximate rate MUST be closer to 150 than 225. You start seeing glucose in urine as soon as you pass threshold (225), but NOT all nephrons have the same transport maximum for glucose. All nephrons have reached their maximal capacity to reabsorb glucose at 300. A 225 could be 1%, 5%, 50% or any number of nephrons, but at 300 we have the 100% of the nephrons, so it must be some number between 150 to 225, but definitely closer to 150. This question is from Guyton Physiology Last edited by bebix; 07072011 at 05:39 AM. Reason: typo 
#22




Thanks!
@ bebix....
thanks for your explaination. I was thinking too much about it so failed to see the overall picture. I also figured out now, that once you get the filtered load of glucose 150 mg/ml. Plot that point on X axis and draw a perpendicular from X axis and you will see it intersect at a point after the Transport Max (Tm) soo the glucose conc in the tubules has definitely passed 300 mg/ml. Great Questions! and thanks once again for the explaination 
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bebix (07072011) 
#23




Quote:

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bebix (07072011) 
#24




Reaffirming Explanation
I am just passing by reaffirming explanation of fellow colleagues.
So you are given GFR= Glomerular filtration Rate which is 20% of your renal Plasma Flow entering the bowman's capsule in a normal healthy individual GFR= 150 ml/min and you are given Px= Concentration of solute in plasma measured in mg/ml Px= 300mg/dl deciliters are equal to 100ml so, Px= 300mg/100ml= 3mg/ml So you can calculate the Filtered Load or Filtration Rate Filtered Load= 150ml/min x 3mg/ml= 450mg/min this is the concentration of glucose entering per minute into bowman's capsule They are asking you about the glucose excretion rate you know two formulas that are used to find excretion rate Renal clearance of inulin or creatine=GFR GFR= Excretion Rate/Px GFR= (Ux)x(V)/Px where Ux is the urinary concentration in plasma and v is the urinary flow rate You are not given the urinary flow rate, but in a normal healthy individual it is usually 1ml/min and most of the time it is given Reabsorption= Filtration  Excretion that you can rearranged where: Excretion= Filtration  Reabsorption In the Graph the point where the first curve reaches a plateau or the line becomes stagnant that is the transport maximum, which is 300 mg/min, you already know the filtration rate so Excretion = 450 mg/min  300 mg/min= 150 mg/min, the reason you dont take the glucose threshold it is because it is a false statement of how much glucose it is excreted since it still continues to reabsorb glucose back to the capillaries, not all of the kidney has reached transport maximum 
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Figures, Physiology, Renal, Step1Questions 
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