Uworld Obstructive uropathy question, mistake???

[medInUSA]

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Dude presents with right flank discomfort, decreased urination over the last week with occasional episodes of high urine output and weakness. On Physical his BP is 140/90 mmHg and his heart rate is 80/min. the serum creatinine level is 2.1 mg/dL. Urinalysis shows few red blood cells, white blood cells, trace protein, and no casts. Which of the following is the most likely cause of his complaints?

Urinary outflow obstruction
glomerulonephritis
inherited renal disease
hematologic malignancy

Answer is Urinary outflow obstruction

their explanation:
"most likely has unilateral obstructive uropathy due to renal calculi. Indications that this is the case include flank pain, which suggests renal capsular distension; his poor urine output, which suggests mechanical obstruction to urine outflow; his intermittent episodes of high volume urination, which can occur when an obstruction is overcome by a large volume of retained urine; and his renal dysfunction (creatinine 2.1 mg/dL), a reflection of pressure atrophy and decreased glomerular filtration secondary to prolonged obstruction."

If they say it is a unilateral obstruction why in the world would the Creatinine rise if the dude still has the other kidney???
according to kaplan intrnal medicine notes page 241 -242
" a large stone in one ureter cannot cause renal failure because creatinine does not rise if there is loss of only one kidney."

One healthy kidney can function enough to maintain GFR and prevent ARF. Thus, as mentioned above, unless the patient only has one kidney or just happens to have bilateral simultaneous tandem coordinated parallel stones causing obstruction, its not going to cause ARF.

mistake???!!!!

 

[RussianJoo]

i don't know, i would think the creatinine would increase.. i mean you took out one kidney it's pretty logical to think that you'll excrete half as much creatinine thus your plasma creatinine will double... i.e. 2.1... it's the same principle as if you double the FiO2 from 21% to 42% your PaO2 will go from about 100 to about 200...

what does a text book like harisson's or a surgical text say?

 

[Scaredshizzles]

i don't know, i would think the creatinine would increase.. i mean you took out one kidney it's pretty logical to think that you'll excrete half as much creatinine thus your plasma creatinine will double... i.e. 2.1... it's the same principle as if you double the FiO2 from 21% to 42% your PaO2 will go from about 100 to about 200...

what does a text book like harisson's or a surgical text say?

I once had a kid with only half a functional kidney and his creatinine was just barely above what it should've been for his age. And presumably it would stay that way for a long time, although chances are with only half a kidney,, over a few decades it will hypertrophy too much and begin losing blood supply/function. And don't foget kidney transplant patients with one kidney usually have CrCl of 75-80% normal at the start....and that's not even a native kidney, that is kidney that has had its arterial supply snapped and reattached after several hours.

 

[medInUSA]

I disagree, it's not a linear relationship like that. If that was the case then how would people donate one kidney while still remaining healthy? No, one kidney is enough to clear your creatinine and BUN.

from harrison's:


"Azotemia develops when overall excretory function is impaired, often in the setting of bladder outlet obstruction, bilateral renal pelvic or ureteric obstruction, or unilateral disease in a patient with a solitary functioning kidney. Complete bilateral obstruction should be suspected when acute renal failure is accompanied by anuria. Any patient with renal failure otherwise unexplained, or with a history of nephrolithiasis, hematuria, diabetes mellitus, prostatic enlargement, pelvic surgery, trauma, or tumor should be evaluated for UTO (urinary tract obstruction)"

 

[Scaredshizzles]

Dude presents with right flank discomfort, decreased urination over the last week with occasional episodes of high urine output and weakness. On Physical his BP is 140/90 mmHg and his heart rate is 80/min. the serum creatinine level is 2.1 mg/dL. Urinalysis shows few red blood cells, white blood cells, trace protein, and no casts. Which of the following is the most likely cause of his complaints?

Urinary outflow obstruction
glomerulonephritis
inherited renal disease
hematologic malignancy

Answer is Urinary outflow obstruction

their explanation:
"most likely has unilateral obstructive uropathy due to renal calculi. Indications that this is the case include flank pain, which suggests renal capsular distension; his poor urine output, which suggests mechanical obstruction to urine outflow; his intermittent episodes of high volume urination, which can occur when an obstruction is overcome by a large volume of retained urine; and his renal dysfunction (creatinine 2.1 mg/dL), a reflection of pressure atrophy and decreased glomerular filtration secondary to prolonged obstruction."

If they say it is a unilateral obstruction why in the world would the Creatinine rise if the dude still has the other kidney???
according to kaplan intrnal medicine notes page 241 -242
" a large stone in one ureter cannot cause renal failure because creatinine does not rise if there is loss of only one kidney."

One healthy kidney can function enough to maintain GFR and prevent ARF. Thus, as mentioned above, unless the patient only has one kidney or just happens to have bilateral simultaneous tandem coordinated parallel stones causing obstruction, its not going to cause ARF.

mistake???!!!!

I agree a stone is not likely to cause rise in creatinine, although he might be dehydrated in addition...In any case, lack of casts speaks against glomerulonephritis, lack of protein speaks against heme malignancy--multiple myeloma, short time course speaks against inherited diseases, lack of major RBCs speaks against something like goodpasture's/other glomerulonephritidies, etc. I guess by exclusion that is the best answer, plus stones are the ones most likely to cause significant acute flank pain.

 

[medInUSA]

Also from Harrison but a different section: (chapter 22)

"POSTRENAL AZOTEMIA
Postrenal azotemia is the least common cause of acute renal failure, accounting for approximately 5–10% of cases, but is important to detect because of its reversibility. It occurs when urinary flow from both kidneys, or a single functioning kidney, is obstructed. Each nephron has an elevated intraluminal pressure, causing a decrease in GFR.

Causes include urethral obstruction, bladder dysfunction or obstruction, and obstruction of both ureters or renal pelvises. In men, benign prostatic hyperplasia is the most common cause. Patients taking anticholinergic drugs are particularly at risk. Bladder, prostate, and cervical cancers as well as retroperitoneal processes and neurogenic bladder can also cause obstruction. Less common causes are blood clots, bilateral ureteral stones, urethral stones or stricture, and bilateral papillary necrosis. In patients with a single functioning kidney, obstruction of a solitary ureter can cause postrenal azotemia."

 

[medInUSA]

I agree a stone is not likely to cause rise in creatinine, although he might be dehydrated in addition...In any case, lack of casts speaks against glomerulonephritis, lack of protein speaks against heme malignancy--multiple myeloma, short time course speaks against inherited diseases, lack of major RBCs speaks against something like goodpasture's/other glomerulonephritidies, etc. I guess by exclusion that is the best answer, plus stones are the ones most likely to cause significant acute flank pain.

I agree with you that it is the best answer choice, but I am pointing out the mistake in the question itself (Cr should have been normal) and also in their explanation (them saying that unilateral obstructive uropathy would cause Cr increase without giving any indication that the other kidney is also impaired)

 

[Scaredshizzles]

Also from Harrison but a different section: (chapter 22)

"POSTRENAL AZOTEMIA
Postrenal azotemia is the least common cause of acute renal failure, accounting for approximately 5–10% of cases, but is important to detect because of its reversibility. It occurs when urinary flow from both kidneys, or a single functioning kidney, is obstructed. Each nephron has an elevated intraluminal pressure, causing a decrease in GFR.

Causes include urethral obstruction, bladder dysfunction or obstruction, and obstruction of both ureters or renal pelvises. In men, benign prostatic hyperplasia is the most common cause. Patients taking anticholinergic drugs are particularly at risk. Bladder, prostate, and cervical cancers as well as retroperitoneal processes and neurogenic bladder can also cause obstruction. Less common causes are blood clots, bilateral ureteral stones, urethral stones or stricture, and bilateral papillary necrosis. In patients with a single functioning kidney, obstruction of a solitary ureter can cause postrenal azotemia."

Yep, this is one of the many uworld questions that are flawed...But the reality is that some of the real test questions are going to be flawed too, you just got to choose the best answer...there is a reason why no one comes anywhere near a perfect score on these exams....it is simply impossible, some of the questions will have errors in them.

 

[medInUSA]

Confirmation from yet another source:

from "USMLE master the boards Step 3" by Conrad Fischer

page 228

" The obstruction must be bilateral to cause renal failure. Unilateral Obstruction cannot cause renal failure"

 

[Scaredshizzles]

Confirmation from yet another source:

from "USMLE master the boards Step 3" by Conrad Fischer

page 228

" The obstruction must be bilateral to cause renal failure. Unilateral Obstruction cannot cause renal failure"

Did they give the BUN in the question though? It is still possible that he was simply dehydrated, as patients in pain (who therefore are not eating/drinking well) often will be. That may have been the point they were making (not sure).

 

[medInUSA]

Did they give the BUN in the question though? It is still possible that he was simply dehydrated, as patients in pain (who therefore are not eating/drinking well) often will be. That may have been the point they were making (not sure).

No they did not give BUN, but in their explanation which I Quoted in my first post they would have mentioned the dehydration, but instead they specifically mention that the Increase in Cr is due to the prolonged renal obstruction.

 

[Scaredshizzles]

No they did not give BUN, but in their explanation which I Quoted in my first post they would have mentioned the dehydration, but instead they specifically mention that the Increase in Cr is due to the prolonged renal obstruction.

You assume the writer of the question and of the explanations is one in the same, but I'm not so sure that is always the case with uworld questions.

 

[agranulocytosis]

I think it's risky to assume all abberant lab values are abnormal without having a baseline to compare the values. If the question stem indicated a large rise in Cr, then I'd feel it's safe to say that the elevated Cr is abnormal.

 

[medInUSA]

I think it's risky to assume all abberant lab values are abnormal without having a baseline to compare the values. If the question stem indicated a large rise in Cr, then I'd feel it's safe to say that the elevated Cr is abnormal.

IN the real world and maybe for UWorld, yes I agree with you that its risky to assume aberrant lab values abnormal without having a baseline.

However for the purposes of the USMLE exam itself, I really don't think they would put that abnormal value there just like that, and therefore I would not ignore any abnormal lab value on the actual exam or assume that that's their baseline.

 

[FirstMANdown]

IN the real world and maybe for UWorld, yes I agree with you that its risky to assume aberrant lab values abnormal without having a baseline.

However for the purposes of the USMLE exam itself, I really don't think they would put that abnormal value there just like that, and therefore I would not ignore any abnormal lab value on the actual exam or assume that that's their baseline.

Sounds like a clear U World mistake, or a incomplete question. Best advice for clarity on this issue that I would recommend is to e-mail UW, give them the exact question and answer and see what explanation they can give you, and more importantly correct the mistake so others who solely rely on UW do not go into the exam with a false since of board preparedness.

 

[medInUSA]

Sounds like a clear U World mistake, or a incomplete question. Best advice for clarity on this issue that I would recommend is to e-mail UW, give them the exact question and answer and see what explanation they can give you, and more importantly correct the mistake so others who solely rely on UW do not go into the exam with a false since of board preparedness.

Agreed, I will email Uworld and let you know their response.

 

[medInUSA]

IN the real world and maybe for UWorld, yes I agree with you that its risky to assume aberrant lab values abnormal without having a baseline.

However for the purposes of the USMLE exam itself, I really don't think they would put that abnormal value there just like that, and therefore I would not ignore any abnormal lab value on the actual exam or assume that that's their baseline.

This on the other hand does not stand true for certain Physical Exam findings which if abnormal can still oftentimes be ignored. For example: Isolated absence of ankle reflexes in the elderly