**Core Concept**
Congenital adrenal hyperplasia (CAH) is a group of inherited disorders that affect the adrenal glands, leading to impaired production of cortisol and aldosterone. The most common cause of CAH is a deficiency of an enzyme involved in the steroidogenesis pathway, specifically 21-hydroxylase.

**Why the Correct Answer is Right**
The enzyme 21-hydroxylase is crucial for converting 17-hydroxyprogesterone to 11-deoxycortisol in the pathway of cortisol synthesis. A deficiency in 21-hydroxylase leads to an accumulation of 17-hydroxyprogesterone, resulting in an overproduction of androgens and a deficiency of cortisol and aldosterone. This enzyme is responsible for approximately 95% of cases of CAH.

**Why Each Wrong Option is Incorrect**
* **Option A:** 11-beta-hydroxylase is another enzyme involved in cortisol synthesis, but it is less common than 21-hydroxylase deficiency and typically presents with hypertension due to an overproduction of deoxycorticosterone.
* **Option B:** 17-alpha-hydroxylase is involved in the synthesis of cortisol and androgens, but its deficiency is rare and typically presents with hypertension and hypokalemia.
* **Option D:** 3-beta-hydroxysteroid dehydrogenase is an enzyme involved in the early steps of steroidogenesis, but its deficiency is rare and typically presents with a more complex clinical picture.

**Clinical Pearl / High-Yield Fact**
Mnemonic to remember the most common enzyme deficiencies in CAH: "21" for 21-hydroxylase, "11" for 11-beta-hydroxylase, and "17" for 17-alpha-hydroxylase.

**Correct Answer: C. 21-hydroxylase**