The first reaction of the Krebs cycle is the combination of acetyl CoA with

Explanation

The two carbon acetyl group is transferred to the four carbon oxaloacetic acid to give the six carbon citric acid, which is why the pathway is also called the citric acid cycle. Citric acid is the product of that step, not the reactant, which is what makes it the tempting answer. Oxaloacetate is regenerated at the end of each turn, so a small amount keeps the cycle running indefinitely.