## **Core Concept**
The question tests the understanding of fatty acid synthesis, specifically the source of reducing equivalents required for this process. Fatty acid synthesis is an anabolic pathway that converts acetyl-CoA and malonyl-CoA into fatty acids, requiring reducing equivalents in the form of NADPH.

## **Why the Correct Answer is Right**
The correct answer, **D. NADPH from the pentose phosphate pathway**, is right because the pentose phosphate pathway (PPP) is a significant source of NADPH in the cell, particularly in tissues involved in lipid synthesis like the liver. NADPH is crucial for providing the reducing power needed for the reduction reactions in fatty acid synthesis. The PPP generates NADPH through the oxidation of glucose-6-phosphate, producing ribulose-5-phosphate and carbon dioxide as byproducts.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while NADH can provide reducing equivalents, it is primarily used in the electron transport chain to produce ATP and is not the primary source for fatty acid synthesis, which requires NADPH.
- **Option B:** This option is incorrect as it does not directly relate to a known major source of reducing equivalents for fatty acid synthesis.
- **Option C:** This option is incorrect because, although it mentions NADPH, it incorrectly associates it with a source that is not primarily linked to fatty acid synthesis in the liver.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the liver is a primary site for fatty acid synthesis, and the pentose phosphate pathway plays a critical role in providing the necessary NADPH for this process. This highlights the importance of understanding metabolic pathways and their interconnectivity in clinical contexts, such as managing metabolic disorders.

## **Correct Answer: D. NADPH from the pentose phosphate pathway**