Final answer:
The TCA cycle begins with the conversion of pyruvate to Acetyl CoA. This is the initial step prior to the series of reactions in the Krebs cycle which ultimately produce ATP, NADH, FADH₂, and CO₂.
Step-by-step explanation:
The TCA cycle, also known as the Krebs cycle or the citric acid cycle, begins with the conversion of pyruvate into acetyl CoA. This is a critical step in cellular respiration and occurs prior to the citric acid cycle. During this conversion, pyruvate, which is a three-carbon molecule derived from glycolysis, is decarboxylated and then combined with coenzyme A to form acetyl CoA, a two-carbon molecule. The carbon that is lost is released as carbon dioxide. This conversion is a link reaction between glycolysis and the Krebs cycle, allowing acetyl CoA to enter the Krebs cycle where it will condense with oxaloacetate to begin the cycle.
The Krebs cycle continues with a series of reactions that produce high-energy molecules such as ATP, NADH, and FADH₂, as well as additional CO₂ as a byproduct. Therefore, the correct answer to the question is Acetyl CoA.