**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 type of CAH is caused by a deficiency of the enzyme 21-hydroxylase, which plays a crucial role in the production of these essential hormones.

**Why the Correct Answer is Right**
21-hydroxylase deficiency accounts for approximately 95% of all CAH cases. This enzyme is responsible for converting 17-hydroxyprogesterone to 11-deoxycortisol in the adrenal glands. Without sufficient 21-hydroxylase activity, the production of cortisol and aldosterone is severely impaired, leading to an overproduction of androgens, such as testosterone and dihydroepiandrosterone (DHEA). This condition can result in virilization of female infants and children, as well as electrolyte imbalances and hypotension due to the lack of aldosterone.

**Why Each Wrong Option is Incorrect**
**Option A:** 11 β Hydroxylase deficiency is another type of CAH, but it is less common than 21-hydroxylase deficiency, accounting for approximately 5% of cases. This deficiency leads to an overproduction of 11-deoxycortisol, which can cause hypertension and hypokalemia.

**Option C:** 17 α Hydroxylase deficiency is a rare form of CAH, characterized by a deficiency of the enzyme that converts pregnenolone to 17-hydroxypregnenolone. This condition can lead to an overproduction of mineralocorticoids and a deficiency of cortisol and sex steroids.

**Option D:** While 21-hydroxylase deficiency is the most common type of CAH, it is not a correct answer to say "none of the above." The other options are indeed types of CAH, although they are less common than 21-hydroxylase deficiency.

**Clinical Pearl / High-Yield Fact**
To remember the most common type of CAH, use the mnemonic "21-Hydroxylase is the most common CAH." This will help you recall the key enzyme involved in the majority of CAH cases.

**✓ Correct Answer: B. 21 Hydroxylase deficiency**