Which of the following is considered a primary substrate for gluconeogenesis?

Lactate is considered a primary substrate for gluconeogenesis because it can be converted into glucose in the liver through a series of enzymatic reactions collectively known as gluconeogenesis. This process is particularly vital during periods of fasting or intense exercise when glucose levels need to be replenished. The conversion begins with lactate being converted to pyruvate by the enzyme lactate dehydrogenase. Once in the form of pyruvate, it can enter the gluconeogenesis pathway to form glucose.

The other options do not serve as primary substrates for gluconeogenesis in the same way. Fructose is metabolized through a different pathway that does not directly enter gluconeogenesis. Glucose is the end product of gluconeogenesis rather than a substrate. Glycogen, while it can be broken down to glucose via glycogenolysis, is not utilized in gluconeogenesis as a direct substrate; rather, it serves as a stored form of glucose. Thus, lactate distinctly fulfills the role of a primary substrate necessary for gluconeogenesis.