**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 starvation. The enzymes involved in gluconeogenesis are often shared with glycolysis, but with opposite catalytic activities.

**Why the Correct Answer is Right**
The enzyme phosphoenolpyruvate carboxykinase (PEPCK) is a key player in gluconeogenesis. It catalyzes the conversion of oxaloacetate to phosphoenolpyruvate (PEP), a crucial step in generating glucose from non-carbohydrate sources. PEPCK is a regulatory enzyme that is induced during fasting and inhibited during feeding, allowing the body to adapt to different energy states. The enzyme uses the energy from GTP to drive the reaction, making it a critical step in gluconeogenesis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glyceraldehyde-3-phosphate dehydrogenase is a glycolytic enzyme that catalyzes the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. It is not involved in gluconeogenesis.
* **Option B:** Phosphofructokinase-1 (PFK-1) is a glycolytic enzyme that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. It is not involved in gluconeogenesis.
* **Option D:** Pyruvate kinase is a glycolytic enzyme that catalyzes the conversion of phosphoenolpyruvate to pyruvate. It is not involved in gluconeogenesis.

**Clinical Pearl / High-Yield Fact**
Gluconeogenesis is a vital process for maintaining blood glucose levels during fasting or starvation. The enzyme PEPCK is a key regulatory step in this process, and its activity is induced during fasting and inhibited during feeding.

**Correct Answer:** C. Phosphoenolpyruvate carboxykinase (PEPCK)