Metabolic Acidosis + High Ammonia, Propionic Acid, and Mehtyl Citrate! - USMLE Forums
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  #1  
Old 10-02-2011
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Biochemistry Metabolic Acidosis + High Ammonia, Propionic Acid, and Mehtyl Citrate!

Laboratory tests on a sick child reveal a low white blood cell count, metabolic acidosis, increased anion gap, and mild hyperammonemia. Measurement of plasma amino acids reveals elevated levels of glycine, and measurement of urinary organic acids reveals increased amounts of propionic acid and methyl citrate. Which of the following processes is most likely?

A. Diabetes mellitus
B. A fatty acid oxidation disorder
C. Vitamin B12 deficiency
D. Propionic acidemia
E. A disorder in glycine catabolism
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I'm going to go with C.

Deficiency in vit. B12 impairs the final step in the synthesis of Succynil Coa from Propionyl Coa, leading to the accumulation of methylmalonate. Hence, the metabolic acidosis with increased anion gap.

Also explains the low WBC count (since we don't have vit. B12, we cannot convert N5-MethylTHD into N5,n10- MethyleneTHD, not being able to produce dTMP from dUMP. So the rapid division rate of these cells is halted by lacking deoxynucleotides)

Glycine is the precursor of ALA along with succynil CoA, and it is increase because there's little availability of the latter.

I'm not so sure about the hyperammonemia. This is just me thinking, so please correct me if I'm wrong. Excess of propionyl Coa takes it through a different pathway, using Propionyl and Oxalacetate to synthesize Methyle Citrate. If there's consumption of oxalacetate in the liver, we might have deficient aspartate to join the urea cycle. And that would cause the mild hyperammonemia.

Great question, very integrative!
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Old 10-02-2011
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I think its D) propionic acidemia
see this http://en.wikipedia.org/wiki/Propionic_acidemia
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Quote:
Originally Posted by mayursn39 View Post
I think its D) propionic acidemia
see this http://en.wikipedia.org/wiki/Propionic_acidemia
Might as well be right. They don't say anything about methylmalonate, or nutritional deficits. And they might want us to think that glycine accumulates from excess threonine, that can't be metabolized further from propionyl coa
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Old 10-02-2011
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The answer is: D
Propionic acidemia results from a block in propionyl CoA carboxylase (PCC), which converts propionic to methylmalonic acid. Excess propionic acid in the blood produces metabolic acidosis with a decreased bicarbonate and increased anion gap (the serum cations sodium plus potassium minus the serum anions chloride plus bicarbonate). The usual values of sodium (~140 meq/L) plus potassium (~4 meq/L) minus those for chloride (~105 meq/L) plus bicarbonate (~20 meq/L) thus yield a normal anion gap of ~20 meq/L. A low bicarbonate of 6 to 8 meq/L yields an elevated gap of 32 to 34 meq/L, a "gap" of negative charge that is supplied by the hidden anion (propionate in propionic acidemia). Biotin is a cofactor for PCC and its deficiency causes some types of propionic acidemia. Vitamin B12 deficiency can cause methylmalonic aciduria because vitamin B12 is a cofactor for methylmalonyl coenzyme A mutase. Glycine is secondarily elevated in propionic acidemia, but no defect of glycine catabolism is present.
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i think it is c
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Old 10-07-2011
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very good question mate.. but why is the WBC count low in propionic acidemia??? please explain..
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Old 10-07-2011
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D. Propionic acidemia
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