## **Core Concept**
The question tests the knowledge of enzymes involved in lipid metabolism, specifically those related to cholesterol and ketone body synthesis. Both cholesterol and ketone bodies are synthesized in the liver and play critical roles in energy metabolism and cellular structure. The enzymes involved in these pathways often share common steps or precursors.

## **Why the Correct Answer is Right**
The correct answer, **HMG-CoA synthase**, is a key enzyme in the synthesis of both cholesterol and ketone bodies. In cholesterol synthesis, HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) is a crucial intermediate formed from acetyl-CoA and acetoacetyl-CoA by the action of HMG-CoA synthase. This HMG-CoA is then reduced to form mevalonate, a step catalyzed by HMG-CoA reductase, which is a major regulatory step in cholesterol synthesis. For ketone body synthesis, during states of low food intake, fatty acid breakdown increases, producing acetyl-CoA. Two molecules of acetyl-CoA combine to form acetoacetyl-CoA, which then combines with another molecule of acetyl-CoA to form HMG-CoA by the action of HMG-CoA synthase. HMG-CoA is then cleaved to form acetoacetate, the first ketone body.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Acetoacetyl-CoA thiolase** is involved in the pathway of ketone body synthesis and can act in the cholesterol biosynthesis pathway by forming acetoacetyl-CoA from two acetyl-CoA molecules, it is not the enzyme that directly participates in both pathways as specifically as HMG-CoA synthase does.
- **Option B:** **Mevalonate kinase** is specifically involved in the cholesterol biosynthesis pathway downstream of HMG-CoA reductase. It phosphorylates mevalonate to form phosphomevalonate and is not directly involved in ketone body synthesis.
- **Option D:** **Pyruvate dehydrogenase** is crucial for the conversion of pyruvate into acetyl-CoA, which can then enter both the citric acid cycle and, indirectly, the pathways for cholesterol and ketone body synthesis. However, it is not directly involved in either cholesterol or ketone body synthesis.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is that **HMG-CoA reductase inhibitors (statins)** are used to lower cholesterol levels by inhibiting the cholesterol synthesis pathway at the HMG-CoA reductase step. This action can indirectly affect ketone body production, but the primary use of statins is to manage hypercholesterolemia.

## **Correct Answer:** .