Commonest cause of congenital adrenal hyperplasia is
**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, and excess production of androgens. The disease is caused by mutations in genes encoding enzymes involved in the steroidogenesis pathway.
**Why the Correct Answer is Right**
The most common cause of CAH is a deficiency of 21-hydroxylase, an enzyme that catalyzes the conversion of 17-hydroxyprogesterone to 11-deoxycortisol in the steroidogenesis pathway. This deficiency leads to a buildup of 17-hydroxyprogesterone, which is shunted towards the production of androgens, resulting in virilization of affected females. The 21-hydroxylase deficiency is responsible for approximately 90-95% of cases of CAH.
**Why Each Wrong Option is Incorrect**
**Option A:** 3-beta-hydroxysteroid dehydrogenase (3-beta-HSD) deficiency is a rare cause of CAH, affecting the conversion of pregnenolone to progesterone and 17-hydroxypregnenolone to 17-hydroxyprogesterone. It is not the most common cause of CAH.
**Option B:** 11-beta-hydroxylase deficiency is another rare cause of CAH, affecting the conversion of 11-deoxycortisol to cortisol. It is characterized by hypertension and hypokalemia due to the accumulation of 11-deoxycorticosterone.
**Option C:** 17-alpha-hydroxylase deficiency is a rare cause of CAH, affecting the conversion of pregnenolone to 17-hydroxypregnenolone and progesterone to 17-hydroxyprogesterone. It is characterized by hypertension and hypokalemia due to the accumulation of mineralocorticoids.
**Clinical Pearl / High-Yield Fact**
The most common presenting feature of CAH in females is virilization, which can include clitoral enlargement, hirsutism, and male-pattern baldness. Early diagnosis and treatment are crucial to prevent long-term consequences of untreated CAH.
**Correct Answer:** C. 21-hydroxylase deficiency is the most common cause of congenital adrenal hyperplasia.