Enzyme common to cholesterol synthesis and ketone body synthesis:
**Core Concept**
The question revolves around the intersection of cholesterol and ketone body synthesis pathways, highlighting the role of a specific enzyme that participates in both processes. This enzyme is crucial for the regulation of lipid metabolism and energy production in the body.
**Why the Correct Answer is Right**
The correct answer is HMG-CoA synthase. This enzyme catalyzes the condensation of acetoacetyl-CoA and acetyl-CoA to form 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA), which is a key intermediate in both cholesterol synthesis (via the mevalonate pathway) and ketone body synthesis (via the ketogenesis pathway). HMG-CoA synthase is a rate-limiting enzyme in both pathways, and its activity is tightly regulated to ensure the optimal production of cholesterol and ketone bodies.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not directly involved in both cholesterol and ketone body synthesis. While it is a critical enzyme in lipid metabolism, it is not the correct answer to this question.
**Option B:** This option is incorrect because it is primarily involved in the breakdown of fatty acids, rather than the synthesis of cholesterol or ketone bodies.
**Option C:** This option is incorrect because it is a key enzyme in the citric acid cycle, but not directly involved in cholesterol or ketone body synthesis.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that HMG-CoA synthase is a key regulatory point in both cholesterol and ketone body synthesis, and its activity is influenced by various factors, including dietary intake, hormonal changes, and genetic mutations.
**Correct Answer:** C. HMG-CoA synthase