KapQ Kidney

Prep for USMLE Forum

» Log in | Register

Forum

Step 1

Match

Prep4USMLE Forum » USMLE Step 1 Forum » Pathology Forum

KapQ Kidney

Author

15 Posts

Lim
Forum Elite
avatar

Topics: 62
Posts: 210

09/06/06 - 08:40 PM

A 45-year-old man suffering from glomerulonephritis has a creatinine clearance of 50ml/min. His medical records indicate that his creatinine clearance was 100 mL/min about 1 year ago. Assuming that there has been no change in his diet, which of the following changes can be expected in this patient compared to 1 year ago?

A. A 2-fold decrease in blood urea nitrogen concentration
B. A 2-fold decrease in creatinine excretion rate
C. A 2-fold increase in creatinine excretion rate
D. A 2-fold increase in creatinine reabsoption
E. A 2-fold increase in plasma creatinine concentration

___________________
The secret is never give up

docsrinu
Forum Elite

Topics: 26
Posts: 213

09/06/06 - 08:47 PM

GFR halved. so plasma creatinine conc should double so that the xcretion rate is constant.

Leopard
Forum Guru
avatar

Topics: 30
Posts: 401

09/06/06 - 09:07 PM

docsrinu wrote:
GFR halved. so plasma creatinine conc should double so that the xcretion rate is constant.

Docsrinu is correct

Extend the thread

How much creatinine is filtered and how much secreted ??

___________________
FA is just a good revision book.It is not a "real" learning tool.

Lim
Forum Elite
avatar

Topics: 62
Posts: 210

09/06/06 - 10:28 PM

yes,you're right(it's E)

excretion rate is Ucr*V.isn't it? so it's constant? why is excretion rate constant?

Ucr*V=Pcr*GFR
GFR is haved,so Pcr is doubled.
but can you explain the left side,'Excretion rate' is constant?
it's intrinsic failure so Excretion rate[amount leaving kidney] is same?

Leopard,they just talked about GFR. so you can't calculate how much,can you?

___________________
The secret is never give up

MD_toronto
Forum Junior

Topics: 2
Posts: 27

09/06/06 - 10:38 PM

Ans E.

This question has couple things that need to be addressed.

first, you should know that creatinine is used as clinical standard for measure of GFR. Although Inulin is gold standard (it's more accurate it is not used in clinincal practice because it's expensive and inulin need to be administered)

Creatinine production = [Cr]plasma * GFR

Normally, Creatinine production remains constant. So, since creatinine level corresponds to GFR. A decrease in creatinine level means equivalent decrease in GFR.

From this question, 1/2 fold decrease creatinine clearance means 1/2 fold decreaase in GFR. And to keep creatinine production constant, there need to be 2 fold increase in plasma creatinine level

1 = 2 * 1/2

Lim
Forum Elite
avatar

Topics: 62
Posts: 210

09/06/06 - 10:54 PM

MD_toronto,thanks.

i missed the fact that total creatinine production is constant.

creatinine production = Ucr*V = Pcr*GFR

so...
even though the patient has a kidney problem... creatinine production = creatinine excretion

at least all (extra) creatinine must go out of body?

if a patient fails to ARF,so Pcr is increasing....so GFR goes down to extreme.
but Ucr*V is same...so...it maybe take long time...anyway it has to go out?

the reason why the patient dies...it takes too long time(or you can say shutdown)???

___________________
The secret is never give up

Leopard
Forum Guru
avatar

Topics: 30
Posts: 401

09/06/06 - 11:13 PM

Thread Extension

plasm Creatine level goes down in which

Physiological conditions
pathological conditions

??????????

___________________
FA is just a good revision book.It is not a "real" learning tool.

Lim
Forum Elite
avatar

Topics: 62
Posts: 210

09/06/06 - 11:39 PM

Leopard wrote:
Thread Extension

plasm Creatine level goes down in which

Physiological conditions pathological conditions

??????????

hm... plasma creatinine goes down???
plasma creatinine concentration is generally constant?
cr comes from muscle and not relatred to food intake.
this is why creatinine can reflect kidney problems.
in dehydration,it can be overestimated...but generally is it constant?

*BUN/Cr decrease
1. Urea systhesis decrease
Low protein diet
Starvation
Liver disease

2. Creatine synthesis increase
Rhabdomyolysis
severe seizure

3. Volume expansion
SIADH
iatrogenic

4.CRF with dialysis

___________________
The secret is never give up

Leopard
Forum Guru
avatar

Topics: 30
Posts: 401

09/06/06 - 11:47 PM

Lim wrote:

hm... plasma creatinine goes down???
plasma creatinine concentration is generally constant?
cr comes from muscle and not relatred to food intake.
this is why creatinine can reflect kidney problems.
in dehydration,it can be overestimated...but generally is it constant?

*BUN/Cr decrease
1. Urea systhesis decrease
Low protein diet
Starvation
Liver disease

2. Creatine synthesis increase
Rhabdomyolysis
severe seizure

3. Volume expansion
SIADH
iatrogenic

4.CRF with dialysis

Your thought is very methodical. Think and try to chalk out the causes about my question ,you will get the answer .

___________________
FA is just a good revision book.It is not a "real" learning tool.

Lim
Forum Elite
avatar

Topics: 62
Posts: 210

09/07/06 - 12:01 AM

#10

so plasma creatinine decrease when GFR increases
Angiotension II increase induce efferent constriction.(GFR increase)

SIADH ( ADH increases plasma volume so dilute )

take too much water (dilute)

hmm... tough...(i feel posting..something wrong.... show me how to think...please)

___________________
The secret is never give up

Leopard
Forum Guru
avatar

Topics: 30
Posts: 401

09/07/06 - 12:13 AM

#11

Plasma concentration x GFR = Constant

Plasma concentration = constant / GFR

So it means

plasma concentration is inversly proportional to GFR

In other words any condition

Which increases GFR will decrease the plasma concentration of Creatinine

The physiological condition which increases GFR = pregnancy
Pathological condition which increases GFR = Diabetese mellitis (early stage)

Other condition which can decrease Plasma concentration of creatinine is

Decreased production when muscle mass of the body is decreased as in malnutrition or old age
In water content (dilutional effect ) as you pointed out = SIADH

Best of luck

___________________
FA is just a good revision book.It is not a "real" learning tool.

ing
Forum Guru
avatar

Topics: 45
Posts: 435

09/07/06 - 09:30 PM

#12

Thanks Leopart. So how to tell how much creatinine is filtered and how much secreted ??

___________________
Never give up!!

ing
Forum Guru
avatar

Topics: 45
Posts: 435

09/07/06 - 09:47 PM

#13

Why not B?

___________________
Never give up!!

ing
Forum Guru
avatar

Topics: 45
Posts: 435

09/07/06 - 09:51 PM

#14

Okay okay, I got it. Cr is freely filtered and not reabsorbed. With the same diet, Cr production rate = cr excretion rate and is constant. Thanks guys.

___________________
Never give up!!

Lim
Forum Elite
avatar

Topics: 62
Posts: 210

09/08/06 - 08:00 AM

#15

ing~YOU GOT IT!!! : )

___________________
The secret is never give up

You don't have permission to post.