**Core Concept**
Hexokinase is a key enzyme involved in the first step of glycolysis, where it catalyzes the conversion of glucose into glucose-6-phosphate (G6P) using ATP as a phosphate donor. This step is crucial for the regulation of glucose metabolism in cells.

**Why the Correct Answer is Right**
Hexokinase is a hexameric enzyme that uses the energy from ATP to phosphorylate glucose at the 6-position, forming glucose-6-phosphate. This reaction is essential for the subsequent steps of glycolysis, and hexokinase is present in various isoforms in different tissues, such as liver (hexokinase I) and muscle (hexokinase II). The unique property of hexokinase is its high affinity for glucose, allowing it to regulate glucose uptake in cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose-6-phosphatase is incorrect because it is an enzyme that catalyzes the reverse reaction of hexokinase, converting glucose-6-phosphate back to glucose.
**Option B:** Phosphofructokinase is incorrect because it is a different enzyme that catalyzes the second step of glycolysis, converting fructose-6-phosphate to fructose-1,6-bisphosphate.
**Option C:** Pyruvate kinase is incorrect because it is an enzyme that catalyzes the final step of glycolysis, converting phosphoenolpyruvate to pyruvate.

**Clinical Pearl / High-Yield Fact**
Hexokinase is a critical enzyme in glucose metabolism, and its dysregulation can lead to various diseases, such as diabetes and cancer. Understanding the role of hexokinase is essential for clinicians to develop effective treatment strategies for these conditions.

**Correct Answer: A. Hexokinase**