**Core Concept**
Oxidative decarboxylation of pyruvate to acetyl CoA is a critical process in cellular respiration, where pyruvate is converted into acetyl CoA by the pyruvate dehydrogenase complex. This process requires the participation of several coenzymes and vitamins, including thiamine pyrophosphate (TPP), lipoic acid, CoA, FAD, and NAD+.

**Why the Correct Answer is Right**
The correct answer is **Niacin (Vitamin B3)**, as it does not directly participate in the oxidative decarboxylation of pyruvate to acetyl CoA. Instead, niacin is involved in the synthesis of NAD+ and NADP+, which are essential for various redox reactions in the cell. However, the direct conversion of pyruvate to acetyl CoA does not require niacin itself.

**Why Each Wrong Option is Incorrect**
**Option A:** Thiamine (Vitamin B1) is incorrect because it is a crucial component of thiamine pyrophosphate (TPP), which acts as a coenzyme for the pyruvate dehydrogenase complex.
**Option B:** Riboflavin (Vitamin B2) is incorrect because it is involved in the synthesis of FAD, which is necessary for the transfer of electrons in the electron transport chain.
**Option C:** Pantothenic acid (Vitamin B5) is incorrect because it is a component of CoA, which is directly involved in the acetylation of coenzyme A during the conversion of pyruvate to acetyl CoA.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the pyruvate dehydrogenase complex is a crucial regulatory point in cellular respiration, and defects in this complex can lead to various neurological disorders, including Leigh syndrome and pyruvate dehydrogenase deficiency.

**Correct Answer:** D. Niacin (Vitamin B3)