**Core Concept**
Fatty acid synthesis is a crucial biochemical process where acetyl-CoA is converted into fatty acids through a series of reactions. This process is essential for the production of triglycerides, phospholipids, and other lipids in the body.

**Why the Correct Answer is Right**
The principle building block in fatty acid synthesis is acetyl-CoA, which donates two-carbon units to the growing fatty acid chain. This process is facilitated by the enzyme acetyl-CoA carboxylase, which converts acetyl-CoA into malonyl-CoA. Malonyl-CoA is then used as a building block for fatty acid elongation, catalyzed by the enzyme fatty acid synthase. This enzyme complex consists of several subunits, each responsible for a specific step in the fatty acid synthesis process.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not directly related to fatty acid synthesis. While ATP is an energy source for various cellular processes, it is not the principle building block in fatty acid synthesis.
**Option B:** This option is incorrect because NADH is a reducing agent involved in various biochemical reactions, but it is not the principle building block in fatty acid synthesis.
**Option C:** This option is incorrect because acetyl-CoA is the principle building block in fatty acid synthesis, not the end product. Fatty acids are the end products of this process.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that fatty acid synthesis is regulated by various factors, including insulin, glucose levels, and the availability of acetyl-CoA. Understanding the biochemical pathways involved in fatty acid synthesis can help you appreciate the complex interplay between metabolic processes in the body.

**Correct Answer: D. Acetyl-CoA. The principle building block in fatty acid synthesis is acetyl-CoA.**