**Core Concept**
Congenital adrenal hyperplasia (CAH) is a group of inherited disorders that affect the adrenal glands, leading to an imbalance of steroid hormones. In this case, the child's presentation of severe dehydration, hyperkalemia, and hyponatremia suggests a salt-wasting form of CAH, which is caused by a deficiency of the enzyme 21-hydroxylase.

**Why the Correct Answer is Right**
Measuring 17-hydroxyprogesterone (17-OHP) levels will be helpful in diagnosing CAH, particularly the salt-wasting form. 17-OHP is a precursor to cortisol and aldosterone, and its elevated levels indicate a block in the conversion to these hormones due to the deficiency of 21-hydroxylase. This block leads to a decrease in aldosterone production, resulting in salt wasting and the subsequent electrolyte imbalances.

**Why Each Wrong Option is Incorrect**
**Option A:** Aldosterone levels would be decreased in CAH due to the deficiency of 21-hydroxylase, not increased.

**Option B:** Cortisol levels may be decreased in CAH, but measuring them would not be as specific or helpful in diagnosing the salt-wasting form as measuring 17-OHP.

**Option C:** Dehydroepiandrosterone sulfate (DHEAS) levels may be decreased in CAH, but this is not as specific or helpful in diagnosing the salt-wasting form as measuring 17-OHP.

**Clinical Pearl / High-Yield Fact**
In CAH, the salt-wasting form is more common in males and is often associated with severe dehydration, hyperkalemia, and hyponatremia. This is because the deficiency of 21-hydroxylase leads to a decrease in aldosterone production, resulting in excessive loss of sodium and water.

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