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Old 10-01-2010
ashishkabir's Avatar
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Biochemistry Urea vs. Uric acid

I know in the heat of things we get these two mixed up - but Lets make sure we remember which is which for example, which accumulates in Lesch Nyhan syndrome or any other conditions...

I think Lesch Nyhan is Uric acid - can someone confirm? I get confused because OTC deficiency and hyperammonemia can also have neurological symptoms and I think those might be urea - am I wrong?
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Old 10-01-2010
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Yes, in Lesch Nyhan disease, there's accumulation of uric acid due to HGPRT deficiency. A characteristic feature is self-mutilation, and it's XR, so it occurs mostly in boys. There's MR & choreoathetosis. My own mnemonic for this disease is pretty crazy, and may not be useful much to a lot of other people. I lived in the UAE for some time, and Nhyan is a pretty common native name there. People there are also known to feast on whole goats roasted on flames in the desert. So, Nhyan + lots of meat = lots of uric acid & gout -- Sorry, if this is offensive to anyone.

In OTC deficiency (XR also), what accumulates is pretty similar to the name of the deficiency; OroTiC acid. It's actually a metabolite of the accumulated carbamoyl phosphate. It appears in blood and urine. Since OTC is the enzyme responsible for the first step of the urea cycle, hyperammonemia also ensues and neurological symptoms are, thus, possible.

In hyperammonemia, there's excess ammonia (NH3), not uric acid. Normally, in the urea cycle, ammonia is converted to urea (not uric acid). Usually, hyperammonemia ensues due to deficiencies in the first step in the urea cycle. It's common in end-stage liver disease, and presents with flabby tremor of the arm (astrexis), due to ammonia being toxic to the brain.
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Old 10-03-2010
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So uric acid goes into the brain and causes self mutilation, MR and choreoathetosis and
ammonia goes in to the brain and causes flapping tremor.

Urea then, is a good thing because if you can't make it then you've got a problem with the urea cycle and ammonia will accumulate.

However if Urea is increased, then depending on whether it's >15X the creatinine clearance in urine there's a kidney problem.

I get a little confused on this part too -

If BUN in urine is >15X creatinine in urine then the problem is with the kidney a nephrotic or nephritic syndrome or ATN or ARF or CRF etc...

If BUN is high but Bun / Creatinine is <15 then the problem is volume to the kidney is low for some reason - such as renal artery stenosis or CHF or something else

Is that right?
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Old 10-03-2010
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Quote:
Originally Posted by ashishkabir View Post
If BUN in urine is >15X creatinine in urine then the problem is with the kidney a nephrotic or nephritic syndrome or ATN or ARF or CRF etc...

If BUN is high but Bun / Creatinine is <15 then the problem is volume to the kidney is low for some reason - such as renal artery stenosis or CHF or something else

Is that right?
No, you mixed things up here <a sign that you need a quick FA fix!> . Holding FA right now, specifically on page 469, I can summarize it as shown below:
  • It's serum BUN/Cr, not urine's.
  • A ratio <15 definitely means a renal issue (e.g. ATN). This is because BUN reabsorption is impaired, and so it's lost in urine.
  • A ratio >15 can mean a prerenal or a postrenal problem. To differentiate the two from each other, prerenal problems are usually associated with hypovolemia or decreased RBF, so urine osmolarity is high (i.e. concentrated urine) due to less water lost, not due to excess solute loss (because Na is retained as well).
EDIT: reading your post again, I can see that you're still correct, cause you reversed both the serum and urine ratios and their corresponding causes, but I think it's better to go with the serum ratio, because that's how it will likely show up on the exam.
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