MC deficient enzyme in Congenital adrenal hyperplasia: March 2013
**Core Concept**
Congenital adrenal hyperplasia (CAH) is a group of inherited disorders that result from the deficiency of one of the enzymes necessary for cortisol production from cholesterol in the adrenal glands. The most common cause is the deficiency of the **21-hydroxylase** enzyme. This deficiency leads to an accumulation of precursors that are then shunted into the production of androgens, resulting in virilization.
**Why the Correct Answer is Right**
The correct answer is related to the most common form of CAH, which is caused by the deficiency of **21-hydroxylase**. This enzyme is crucial for converting **17-hydroxyprogesterone** to **11-deoxycortisol** in the cortisol biosynthesis pathway. Without it, the production of cortisol and aldosterone is impaired, leading to an increase in the production of androgens.
**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not correspond to the most common enzyme deficiency associated with CAH.
**Option B:** Similarly, this option is incorrect as it is not related to the primary enzymatic defect in CAH.
**Option C:** This choice is also incorrect for the same reason, not being the enzyme primarily associated with CAH.
**Option D:** While other enzymes can be deficient in different forms of CAH, **21-hydroxylase** deficiency is the most common.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that **21-hydroxylase deficiency** accounts for about 90-95% of CAH cases, making it a critical diagnosis to consider in cases of virilization or adrenal insufficiency.
**Correct Answer:** **D. 21-hydroxylase**