In the zona fasciculata, cortisol synthesis requires transport of cholesterol to the inner mitochondrial membrane by which rate-limiting protein?

• A Steroidogenic acute regulatory protein (StAR) ✓
• B Sterol carrier protein-2 (SCP-2)
• C Cytochrome P450 side-chain cleavage enzyme (CYP11A1) itself is the rate-limiting step
• D Peripheral-type benzodiazepine receptor (TSPO) alone controls steroidogenesis rate

Explanation

The rate-limiting step in adrenal steroidogenesis is the transport of cholesterol from the outer to the inner mitochondrial membrane, where CYP11A1 (cholesterol side-chain cleavage enzyme) converts it to pregnenolone. This transport is mediated by StAR (Steroidogenic Acute Regulatory Protein), a rapidly synthesized, rapidly degraded protein whose expression is stimulated by ACTH via the cAMP-PKA pathway. Mutations in StAR cause lipoid congenital adrenal hyperplasia (most severe CAH form) with absent steroidogenesis. TSPO (on outer membrane) facilitates cholesterol transfer but StAR is the classical rate-limiting regulated step.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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