Can't be 17α-hydroxylase because this would present with hypertension and some sort of ambiguous sexual presentation (males) or lack of secondary sexual development (females). Can't be 11β-hydroxylase because this would present with the opposite of the kid's presentation due to the production of 11-deoxycorticosterone (similar effects to aldosterone but not nearly as potent). The other two would not really make sense since there are increased levels of 17-hydroxyprogesterone

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency.

https://en.wikipedia.org/wiki/Congenital_adrenal_hyperplasia_due_to_21-hydroxylase_deficiency

The moment you see hyponatremia, dehydration, and SALT WASTING in an infant, child, or neonate then stop reading the rest of the question and automatically pick 21-hydroxylase deficiency. That should save you like a minute.