**Core Concept**
The rate-limiting enzyme of fatty acid synthesis is a crucial regulatory step in the biosynthesis of fatty acids, which are essential components of cellular membranes and energy-rich molecules. This enzyme plays a pivotal role in controlling the flow of carbon atoms from glucose into the fatty acid synthesis pathway.

**Why the Correct Answer is Right**
The rate-limiting enzyme of fatty acid synthesis is Acetyl-CoA Carboxylase (ACC). ACC catalyzes the carboxylation of acetyl-CoA to malonyl-CoA, which is the committed and most heavily regulated step in fatty acid biosynthesis. This reaction is a key regulatory point, as it commits acetyl-CoA to the fatty acid synthesis pathway rather than allowing it to be used for other purposes. ACC is activated by citrate, which indicates an abundance of building blocks for fatty acid synthesis, and is inhibited by AMP, which indicates a lack of energy.

**Why Each Wrong Option is Incorrect**
**Option A:** While Acetyl-CoA is a crucial substrate in fatty acid synthesis, it is not the rate-limiting enzyme.
**Option B:** HMG-CoA Reductase is the rate-limiting enzyme of cholesterol synthesis, not fatty acid synthesis.
**Option C:** Fatty acid synthase is a multi-enzyme complex that catalyzes the elongation of fatty acids, but it is not the rate-limiting enzyme.

**Clinical Pearl / High-Yield Fact**
The regulation of fatty acid synthesis is closely linked to the regulation of glucose metabolism, as fatty acid synthesis is often upregulated in response to excess glucose availability.

**Correct Answer: A. Acetyl-CoA Carboxylase.**