**Core Concept**
The child's presentation is suggestive of congenital adrenal hyperplasia (CAH), specifically 21-hydroxylase deficiency, which leads to an inability to convert 17-hydroxyprogesterone (17-OHP) to 11-deoxycortisol. This results in an accumulation of 17-OHP, causing virilization in females, and an overproduction of androgens, which can lead to salt-wasting and electrolyte imbalances.

**Why the Correct Answer is Right**
Measuring 17-hydroxyprogesterone (17-OHP) levels will be helpful in confirming the diagnosis of CAH, specifically 21-hydroxylase deficiency. Elevated levels of 17-OHP are a hallmark of this condition, and it can be used as a diagnostic marker. The measurement of 17-OHP levels can be done through a simple blood test.

**Why Each Wrong Option is Incorrect**
**Option A:** Aldosterone levels would be expected to be low in CAH due to the deficiency of 21-hydroxylase, which is necessary for the production of aldosterone.
**Option B:** Cortisol levels would be expected to be low in CAH due to the deficiency of 21-hydroxylase, which is necessary for the production of cortisol.
**Option C:** Dehydroepiandrosterone sulfate (DHEAS) levels can be elevated in CAH due to the overproduction of androgens, but it is not as specific a marker as 17-OHP.

**Clinical Pearl / High-Yield Fact**
In a child presenting with severe dehydration, hyperkalemia, and hyponatremia, CAH should be considered in the differential diagnosis, especially if the child has normal genitalia but exhibits signs of virilization.

**Correct Answer:** C. 17-hydroxyprogesterone (17-OHP)