ACTH stimulates cortisol synthesis in the zona fasciculata. The rate-limiting and rapidly regulated step in steroidogenesis that ACTH controls is:

• A 17-alpha-hydroxylase activity in the endoplasmic reticulum
• B Transport of cholesterol into mitochondria mediated by the steroidogenic acute regulatory protein ✓
• C Conversion of androstenedione to testosterone by 17-beta-HSD
• D Binding of cholesterol esters to LDL receptors on the cell surface

Explanation

The first enzymatic step, cholesterol side-chain cleavage (CYP11A1), sits on the inner mitochondrial membrane, but cholesterol cannot reach it freely. StAR protein shuttles cholesterol across the mitochondrial membranes, and ACTH raises StAR expression acutely via cAMP, making this the rate-limiting regulated step. Option A involves an enzyme present but not the acute control point. Option D describes LDL uptake, a long-term cholesterol supply mechanism, not the acute rate limiter.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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