Cholesterol is eliminated from the body mainly after conversion to bile acids in the liver. The rate-limiting enzyme of this pathway, which is subject to feedback inhibition by bile acids returning through enterohepatic circulation, is:

• A HMG-CoA reductase
• B Sterol 12-alpha-hydroxylase
• C 27-hydroxylase (CYP27A1)
• D Cholesterol 7-alpha-hydroxylase (CYP7A1) ✓

Explanation

Cholesterol 7-alpha-hydroxylase, a microsomal cytochrome P450 enzyme, performs the committed first step converting cholesterol to 7-alpha-hydroxycholesterol on the way to cholic and chenodeoxycholic acids. Bile acids reabsorbed from the ileum feed back to suppress CYP7A1 transcription via FXR signaling. HMG-CoA reductase controls synthesis, not catabolism, and its inhibition does not directly block bile acid formation. CYP27A1 acts later in the side-chain oxidation step and sterol 12-alpha-hydroxylase determines the cholic to chenodeoxycholic ratio.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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