Final answer:
Acetyl CoA enters the citric acid cycle by combining with oxaloacetate to form citrate while releasing carbon dioxide. This process also produces NADH.
Step-by-step explanation:
The acetyl group enters a cyclic sequence of reactions known as the citric acid cycle (or Krebs cycle). Acetyl CoA, which is a two-carbon molecule, combines with a four-carbon molecule called oxaloacetate to form the six-carbon molecule citrate. This process releases carbon dioxide and generates the reduced form of the electron carrier NADH.