**Core Concept**
Ketone body synthesis is a metabolic pathway that occurs in the liver, where it produces ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) from fatty acids. This process is crucial for energy production in tissues during periods of low glucose availability.

**Why the Correct Answer is Right**
The regulation of ketone body synthesis is primarily controlled by the enzyme HMG-CoA synthase, which is inhibited by high levels of ATP and citrate, and activated by low levels of ATP and NADH. Additionally, the transcription factor PPARα (peroxisome proliferator-activated receptor alpha) plays a role in regulating the expression of genes involved in ketone body synthesis, including HMG-CoA synthase. PPARα is activated by high levels of fatty acids and low levels of glucose.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the regulation of ketone body synthesis.
**Option B:** While insulin does play a role in glucose metabolism, it is not directly involved in the regulation of ketone body synthesis.
**Option C:** The enzyme HMG-CoA reductase is actually involved in the synthesis of cholesterol, not ketone bodies.

**Clinical Pearl / High-Yield Fact**
In states of prolonged fasting or starvation, the liver increases the production of ketone bodies to provide energy to the brain and other tissues. This process is known as ketosis.

**Correct Answer: D. HMG-CoA synthase**