**Core Concept**
De novo fatty acid biosynthesis is a complex biochemical pathway that generates fatty acids from simpler molecules. This process starts in the cytosol with citrate, which is then converted into acetyl-CoA. Subsequent steps involve the assembly of a fatty acid chain through a series of elongation reactions.

**Why the Correct Answer is Right**
The correct answer, **Acetyl-CoA carboxylase (ACC)**, is the enzyme responsible for the first committed and rate-limiting step in de novo fatty acid biosynthesis. ACC catalyzes the carboxylation of acetyl-CoA to form malonyl-CoA, which then enters the fatty acid synthase complex to initiate fatty acid chain elongation. This step is crucial for the regulation of fatty acid biosynthesis, as it is the point at which the pathway is tightly controlled by various signaling molecules.

**Option A:** Acetyl-CoA synthetase is incorrect because it catalyzes the conversion of acetate to acetyl-CoA, but it is not the enzyme responsible for the initial carboxylation step in de novo fatty acid biosynthesis.

**Option B:** Citrate lyase is incorrect because it catalyzes the conversion of citrate to acetyl-CoA, but it is not the enzyme responsible for the subsequent carboxylation step.

**Option D:** Fatty acid synthase is incorrect because it is the enzyme complex that elongates the fatty acid chain, but it is not the enzyme responsible for the initial carboxylation step.

**Clinical Pearl / High-Yield Fact**
The regulation of fatty acid biosynthesis is tightly linked to the regulation of glucose metabolism, particularly in the context of insulin signaling. In insulin-resistant states, such as type 2 diabetes, fatty acid biosynthesis is often dysregulated, leading to increased lipogenesis and insulin resistance.

**Correct Answer:** C. Acetyl-CoA carboxylase (ACC)

✓ Correct Answer: C. NADH is Required - no; Major Product is - Palmitic acid; Occurs at a Glucagon-to-insulin Ratio Best Described as - low; Required Cofactors - Biotin and Coenzyme A