**Core Concept**
Gluconeogenesis is a metabolic pathway that generates glucose from non-carbohydrate sources such as lactate, glycerol, and certain amino acids. This process is crucial for maintaining blood glucose levels during fasting or when glucose is in short supply.

**Why the Correct Answer is Right**
The correct substrate for gluconeogenesis is pyruvate, which is derived from the breakdown of glucose and other carbohydrates. Pyruvate is converted to oxaloacetate by the enzyme pyruvate carboxylase, and then to phosphoenolpyruvate by the enzyme phosphoenolpyruvate carboxykinase. This pathway is essential for maintaining blood glucose levels, particularly during fasting or when glucose is in short supply. The key enzymes involved in this pathway include pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and fructose-1,6-bisphosphatase.

**Why Each Wrong Option is Incorrect**
**Option A:** Lactate is a substrate for gluconeogenesis, but it is not the most direct or primary substrate. Lactate is converted to pyruvate by the enzyme lactate dehydrogenase, which is then fed into the gluconeogenic pathway.
**Option B:** Glycerol is a substrate for gluconeogenesis, but it is not the primary substrate. Glycerol is converted to glyceraldehyde-3-phosphate by the enzyme glycerol kinase, which is then converted to dihydroxyacetone phosphate and eventually to glucose.
**Option C:** Glucose is not a substrate for gluconeogenesis, as it is the end product of the pathway. Gluconeogenesis generates glucose from non-carbohydrate sources, not from glucose itself.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that gluconeogenesis is a critical mechanism for maintaining blood glucose levels, particularly during fasting or when glucose is in short supply. The key enzymes involved in this pathway, such as pyruvate carboxylase and phosphoenolpyruvate carboxykinase, are essential for glucose production.

**Correct Answer:** B. Glycerol