During prolonged fasting, hepatocytes actively synthesize ketone bodies and export them into blood. These same hepatocytes cannot oxidize their own ketone bodies because liver lacks which enzyme?

• A Mitochondrial HMG-CoA synthase
• B Beta-hydroxybutyrate dehydrogenase
• C Acetoacetyl-CoA thiolase
• D Succinyl-CoA:3-ketoacid CoA transferase (thiophorase) ✓

Explanation

Activation of acetoacetate to acetoacetyl-CoA in extrahepatic tissues requires succinyl-CoA:3-ketoacid CoA transferase (SCOT, thiophorase), which transfers CoA from succinyl-CoA. Liver expresses the full ketogenic machinery but lacks SCOT entirely, making it a net exporter of ketones and preventing a futile cycle. Acetoacetyl-CoA thiolase and beta-hydroxybutyrate dehydrogenase are present in liver and function in ketogenesis itself, so they are not the missing step.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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