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Retired NBME Free 120 Answers
namesthegame22
Nice! Yes, the NBT test is normal with MPOD. If it says, "A nitroblue tetrazolium test shows deficient reduction by granulocytes" = CGD!
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Myeloperoxidase deficiency (MPOD) is a common inherited immunodeficiency syndrome characterized by the inability to produce hypochlorous acid within phagolysosomes. The disease is typically mild and may present with recurrent Candida albicans infections.
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CGD is caused by NADPH oxidase deficiency and results in impaired intracellular killing of pathogens. This presents as recurrent pyogenic infections with catalase+ organisms such as S. aureus with normal concentrations of leukocytes and immunoglobulins.
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thekneesofbees
Its because SOD forms H2O2, but H2O2 is broken down by catalase. Since Staph Aureus is catalase positive, it wouldn't be killed by it anyway.
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minion7
@thekneesofbees ,,,,, so if patient has SOD DEFICIENCY, he cant generate H202,, so he is much more susceptible to get killed by organism??
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topgunber
The first thing to keep straight: reactions in WBCs are meant to increase R.O.S. while reactions in other cells are to get rid of R.O.S. superoxide is much stronger than h2o2 (and is what we're testing with the respiratory burst test for CGD-NADPH Oxidase deficiency). Side note: SOD deficiency would manifest as lou gehrig's disease (ALS) because superoxide is so toxic it damages the cells themselves.After SOD works on O, it makes H2O2. Myeloperoxidase takes H2O2 and turns it into bleach (HOCl-). The question is implying that BOTH candida and staph cannot be killed by this patient. They are both catalase positive- this means that if the patient can't turn H2O2 into the stronger HOCl- then the catalase positive organisms can just turn h2o2 into water and oxygen. hope that makes sense.
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b1ackcoffee
I thought SOD also comes (second) after pathway (making h2o2) to make HOโCl
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mdmofongo
Now that I took a look at the pathway again you are right. Knowing this Iโd say that free radicals of O2 still have a chance to cause some damage, as where Hydrogen peroxide just serves as a intermediate for free radical formation. Thus SOD deficiency leaves some bactericidal action but MPO leaves none?
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igurl11
So I missed this too bc I was looking for NADPH Ox (bc I interpreted delayed action against Staph A as NO effect on catalase +). Then briefly considered MPO then picked SOD and moved on. But MPO makes sense bc its needed to make Bleach (how neutrophils kill) -> Def => Candida infection. What they mean by delay in activity for Staph is talking about the extra step needed to use the bacteria's own H2O2 to then make Bleach... But just as the bacteria is making H2O2, it's also removing it using catalase. So it take a little longer to have enough bleach to have a bactericidal effect.
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csalib2
heads up, i think youโre in the wrong question thread
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submitted by โitsalwayslupus(48)
From Pathoma Ch. 2 pg. 14: MPO deficiency results in defective conversion of H2O2 to HOCl. Increased risk for Candida infxns (although most patients asymptomatic). Nitroblue tetrazolium test is normal because the respiratory burst is actually still intact. This patient has candida infection and delayed, but still can kill staph a, which seemingly goes along with the fact that the respiratory burst is still intact, but without MPO it may be hindered a little.