NBME 18 21-Hydroxylase (NBME Answers)

Retired NBME 18 Answers

nbme18/Block 1/Question#17 (reveal difficulty score)
4 day old boy, vomited throughout night after ...
21-Hydroxylase 🔍 / 📺 / 🌳 / 📖
tags: CAH endo

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submitted by t0pcheese(9)

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why is the Na normal in this patient? Everything else made sense, the high K and 17 hydroxyprogesterone.

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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. +

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submitted by ∗focus(45)

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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.

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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) +2

boxboys also increased 17-hydroxyprogesterone is a marker for 21-hydroxylase deficiency (FA2020 p 335) +

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submitted by ∗an1(114)

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21-OH is the most common def, and it has 3 subtypes.

1) A Classic salt wasting: at birth, dehydrated, low Aldosterone (high K, low H, low water), ambiguous genitalia

2) A classic non salt wasting: at birth, ambiguous genitalia, fluid level normal

3) non classic late subtype which is basically A/S and just has girls with acne, oligomenorrhea and hirsutism.

High 17 indicates that the defect must be 21.

11 defects --> 21 will be high because 11 comes after 21, hypertension will be seen

watch Dr. Randy Neils YouTube video on this, amazing job!

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