Hello,

The substrates for glycolysis are glucose, fructose and galactose, which are monosaccharides. Some AA can enter glycolysis (such as alanine, glycine and threonine) but this is beyond the scope of the MCAT. Lipids do not directly enter glycolysis, but glycerol backbones can enter as substrates of glycolysis. For the purposes of the MCAT, it is sufficient to know that monosaccharides are the main substrates of glycolysis, while lipids/AA enter the citric acid cycle.

For how to avoid the vague wording of some questions, it can be helpful to see the context of the passage... in this case, the passage described how epithelial cells decrease inflammation using glycolysis. Looking at the graph, we see that the substrate for glycolysis in this case is glucose (on the Y-axis) and in the passage title (at the end in small font), it says that the article focuses on carbhoydrate metabolism, confirming that this passage also treats glucose (monosaccharides) as the main substrate for glycolysis.