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NBME 18 Answers

nbme18/Block 1/Question#17 (15.9 difficulty score)
4 day old boy, vomited throughout night after ...

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submitted by focus(28),

So we already know the answer is either 11- or 21- Betahydroxylase. How to determine which one?

Question stem cells "dry mucous membranes". He is dehydrated.

We already know that he can't make aldosterone since the lack of either of these enzymes would block that pathway. However, if he is able to make a molecule that ACTS like aldosterone, he would not have dry mucus membranes. He could save water.

Do we have such a molecule? YES! 11-deoxycorticosterone.

Now, if he has 11-beta hydroxylase deficiency, there would be a buildup of 11-deoxycorticosterone. Hence, it has to be the other option.

focus  Not to mention, 21-hydroxylase is the more common one! +2  
boxboys  also increased 17-hydroxyprogesterone is a marker for 21-hydroxylase deficiency (FA2020 p 335) +  
boxboys  also increased 17-hydroxyprogesterone is a marker for 21-hydroxylase deficiency (FA2020 p 335) +  

submitted by t0pcheese(5),
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aneurysmclip  could be due to increased blood volume leading to secretion of ANP and thus natriuresis which would normalize serum sodium levels? that's the only reason I could think of +  
dysdiadochokinesia  My guess is that the sodium and water loss from 21a-hydroxylase deficiency would result in increased RAAS activation and production of Angiotensin II (AGII). Recall that AGII affects the PCT by upregulating Na/H+ exchangers, allowing for there Na/H2O to be resorbed, thus partially counteracting the impact of aldosterone deficiency at the collecting duct. However, I'm not sure if the effect of AGII on the PCT is great enough to entirely counterbalance the loss of aldosterone to present with normal sodium levels. +