**Core Concept**
Acetyl CoA is a crucial molecule in cellular metabolism, serving as a key substrate for various enzymatic reactions, including those involved in the citric acid cycle, fatty acid synthesis, and cholesterol synthesis. It is synthesized from pyruvate in the mitochondria and plays a central role in the regulation of energy metabolism.

**Why the Correct Answer is Right**
Acetyl CoA is indeed a substrate for HMG-CoA synthetase, which catalyzes the condensation of acetyl CoA and acetoacetyl CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). It is also a substrate for malic enzyme, which catalyzes the conversion of acetyl CoA to malate in the presence of NADP+. Furthermore, acetyl CoA is a substrate for malonyl CoA synthetase, which catalyzes the formation of malonyl CoA from acetyl CoA and ATP. However, acetyl CoA is not a direct substrate for fatty acid synthetase, which instead uses acetyl-CoA as a primer to initiate fatty acid synthesis.

**Why Each Wrong Option is Incorrect**
**Option A:** HMG-CoA synthetase indeed uses acetyl CoA as a substrate, so this option is incorrect.
**Option B:** Malic enzyme also uses acetyl CoA as a substrate, making this option incorrect.
**Option C:** Malonyl CoA synthetase uses acetyl CoA as a substrate, so this option is incorrect.

**Clinical Pearl / High-Yield Fact**
Acetyl CoA is a key regulator of energy metabolism and is involved in the synthesis of cholesterol, fatty acids, and the citric acid cycle. It is also a critical substrate for the synthesis of ketone bodies, which are produced in the liver and used as an energy source during periods of fasting or low carbohydrate intake.

**Correct Answer:** D. Fatty acid synthetase.