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.