Final answer:
Acetyl-CoA is the intermediate that connects the oxidation pathways of various food molecules such as carbohydrates, fats, and proteins by entering the citric acid cycle.
Step-by-step explanation:
The intermediate that links the breakdown of food molecules from different starting points is Acetyl-CoA. Acetyl-CoA serves as a central intermediary in the oxidation pathways of carbohydrates, fats, and proteins. During the catabolic processes, these macromolecules are broken down into acetyl-CoA, which then enters the citric acid cycle. The citric acid cycle processes the acetyl-CoA and produces NADH, FADH2, and a precursor of ATP, linking various food molecule metabolic pathways. Thus, Acetyl-CoA is the correct intermediate that unifies the breakdown of different food molecules.