What would be the rate of urinary glucose excretion? - USMLE Forums
 USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
 Home USMLE Articles USMLE News USMLE Polls USMLE Books USMLE Apps
 USMLE Forums What would be the rate of urinary glucose excretion?

 USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam

#1
07-05-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,361 Times in 900 Posts Reputation: 3371
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.

x-axis should read 100,200 & 300

Last edited by bebix; 07-05-2011 at 01:57 PM. Reason: x-axis should read 100,200 & 300
 The above post was thanked by: Abdulaziz (07-27-2011), amnios (07-09-2011), Claus_CU (07-06-2011), dr_lizard (07-07-2011), INCOGNITO (07-05-2011), jahn77 (07-05-2011), parichi (06-24-2012), pass7 (07-05-2011), rulz (07-05-2011), struggle (07-05-2011)

#2
07-05-2011
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 124 Threads: 5 Thanked 217 Times in 73 Posts Reputation: 227
Ans.

I choose A. 0mg/min.The transport maximum for glucose is normal, so there would be no glucose excretion.
 The above post was thanked by: bebix (07-05-2011)
#3
07-05-2011
 USMLE Forums Master Steps History: 1+CK+CS Posts: 735 Threads: 89 Thanked 315 Times in 207 Posts Reputation: 327

yes i think its A...
 The above post was thanked by: bebix (07-05-2011)

#4
07-05-2011
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 124 Threads: 5 Thanked 217 Times in 73 Posts Reputation: 227

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
07-05-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,361 Times in 900 Posts Reputation: 3371

Quote:
 Originally Posted by NGaleas 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.
yup..the threshold is 225. thx
#6
07-05-2011
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 185 Threads: 42 Thanked 111 Times in 62 Posts Reputation: 121

A).i guess
 The above post was thanked by: bebix (07-05-2011)
#7
07-05-2011
 USMLE Forums Master Steps History: 1+CK+CS Posts: 735 Threads: 89 Thanked 315 Times in 207 Posts Reputation: 327

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
07-05-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,361 Times in 900 Posts Reputation: 3371

Quote:
 Originally Posted by confident 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
her is the same pic in jpg format.

click image to enlarge
 The above post was thanked by: Ace3 (07-05-2011)
#9
07-05-2011
 USMLE Forums Master Steps History: 1+CK+CS Posts: 735 Threads: 89 Thanked 315 Times in 207 Posts Reputation: 327

 The above post was thanked by: bebix (07-05-2011)
#10
07-05-2011
 USMLE Forums Master Steps History: 1+CK+CS Posts: 570 Threads: 26 Thanked 554 Times in 276 Posts Reputation: 564

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; 07-05-2011 at 04:31 PM. Reason: Review the post...
 The above post was thanked by: bebix (07-05-2011), struggle (07-05-2011)
#11
07-05-2011
 USMLE Forums Scout Steps History: Not yet Posts: 15 Threads: 1 Thanked 3 Times in 3 Posts Reputation: 13

 The above post was thanked by: bebix (07-05-2011)
#12
07-05-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 5 Threads: 2 Thanked 1 Time in 1 Post Reputation: 11

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
 The above post was thanked by: bebix (07-05-2011)
#13
07-05-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 326 Threads: 20 Thanked 230 Times in 130 Posts Reputation: 242
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 450-225=225 would be excreted....
 The above post was thanked by: docji (07-06-2011), greentrees (07-31-2017)
#14
07-05-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 326 Threads: 20 Thanked 230 Times in 130 Posts Reputation: 242

maybe...if transport is kept until max; than maybe C??? 450-300=150
 The above post was thanked by: bebix (07-05-2011)
#15
07-05-2011
 USMLE Forums Scout Steps History: Step 1 Only Posts: 46 Threads: 8 Thanked 37 Times in 18 Posts Reputation: 47

I think it is A
 The above post was thanked by: bebix (07-05-2011)

#16
07-05-2011
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 333 Threads: 61 Thanked 408 Times in 91 Posts Reputation: 418
(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 300-225 = 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
07-05-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,361 Times in 900 Posts Reputation: 3371

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
 The above post was thanked by: Amenah (07-07-2011), Casandra (06-23-2012), Claus_CU (07-06-2011), donofitaly (07-06-2011), heights (07-06-2011), INCOGNITO (07-05-2011), Lara (07-07-2011), Learner (06-23-2012), leo lakhani (07-07-2011), meldz09 (07-06-2011), neoeinstein (07-05-2011), NMWH (07-05-2011), pass7 (07-05-2011), struggle (07-05-2011)
#18
07-06-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 8 Threads: 0 Thanked 3 Times in 3 Posts Reputation: 13
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
07-06-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,361 Times in 900 Posts Reputation: 3371

Quote:
 Originally Posted by aim_2012 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.
@aim_2012

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
 The above post was thanked by: Claus_CU (07-06-2011), donofitaly (07-06-2011), leo lakhani (07-07-2011)
#20
07-06-2011
 USMLE Forums Addict Steps History: Not yet Posts: 146 Threads: 34 Thanked 37 Times in 27 Posts Reputation: 47
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
07-07-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,361 Times in 900 Posts Reputation: 3371

Quote:
 Originally Posted by docji 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????
@docji
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; 07-07-2011 at 05:39 AM. Reason: typo
 The above post was thanked by: docji (07-07-2011), Lara (07-07-2011), pass7 (07-07-2011)
#22
07-07-2011
 USMLE Forums Addict Steps History: Not yet Posts: 146 Threads: 34 Thanked 37 Times in 27 Posts Reputation: 47
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
 The above post was thanked by: bebix (07-07-2011)
#23
07-07-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 8 Threads: 0 Thanked 3 Times in 3 Posts Reputation: 13

Quote:
 Originally Posted by bebix @aim_2012 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
thanksss a ton......this was really helpful.....
 The above post was thanked by: bebix (07-07-2011)
#24
09-19-2016
 USMLE Forums Scout Steps History: Not yet Posts: 12 Threads: 2 Thanked 2 Times in 2 Posts Reputation: 12
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

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

 Tags Figures-, Physiology-, Renal-, Step-1-Questions

Message:
Options

## Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
User Name:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
 AMG IMG ---
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

 Not yet Step 1 Only CK Only CS Only 1 + CK 1 + CS 1+CK+CS CK+CS 1+CK+CS+3 ---
Favorite USMLE Books
 What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
 Where you live. Leave blank if you don't want to tell.

## Log-in

 Similar Threads Thread Thread Starter Forum Replies Last Post dr 2A USMLE Step 2 CS Forum 2 07-05-2011 11:08 AM bebix USMLE Step 1 Forum 31 06-21-2011 06:58 PM Claus_CU USMLE Step 1 Forum 5 05-25-2011 07:38 PM Dr. N Elham USMLE Step 2 CK Forum 2 06-30-2010 06:12 PM Seetal USMLE Step 1 Forum 3 04-16-2010 05:01 PM

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.

USMLE® & other trade marks belong to their respective owners, read full disclaimer
USMLE Forums created under Creative Commons 3.0 License. (2009-2019)