**Core Concept**
The synthesis of cortisol involves a series of enzyme-catalyzed reactions that convert cholesterol into cortisol. The rate-limiting step in this process is crucial for regulating cortisol production.

**Why the Correct Answer is Right**
The rate-limiting step in cortisol synthesis is catalyzed by the cholesterol side-chain cleavage enzyme (P450scc), which is encoded by the CYP11A1 gene. This enzyme catalyzes the conversion of cholesterol to pregnenolone, a crucial step in the biosynthesis of all steroid hormones, including cortisol. The cholesterol side-chain cleavage enzyme is located in the mitochondria and is regulated by ACTH (adrenocorticotropic hormone), which stimulates the production of cortisol.

**Why Each Wrong Option is Incorrect**
* **Option A:** 21-Hydroxylase is involved in the conversion of 17-hydroxyprogesterone to 11-deoxycortisol, which is an intermediate step in cortisol synthesis, but it is not the rate-limiting step.
* **Option B:** 3b-Hydroxysteroid dehydrogenase is involved in the conversion of pregnenolone to progesterone, but it is not the rate-limiting step in cortisol synthesis.
* **Option D:** 11b-Hydroxylase is involved in the conversion of 11-deoxycortisol to cortisol, but it is not the rate-limiting step in cortisol synthesis.

**Clinical Pearl / High-Yield Fact**
Deficiency of the cholesterol side-chain cleavage enzyme can lead to congenital adrenal hyperplasia, a condition characterized by impaired cortisol production and excessive production of mineralocorticoids.

**✓ Correct Answer: C. Cholesterol side-chain cleavage enzyme**