**Core Concept**
Glycolysis is the first step in cellular respiration, where glucose is broken down into pyruvate, generating a small amount of ATP and NADH. The key regulatory enzyme of glycolysis ensures that the process is tightly controlled and adjusted according to the cell's energy needs.

**Why the Correct Answer is Right**
The key regulatory enzyme of glycolysis is hexokinase, which catalyzes the conversion of glucose to glucose-6-phosphate (G6P). This enzyme is crucial because it commits glucose to the glycolytic pathway, making it unavailable for other cellular processes. Hexokinase is also responsible for the first committed step of glycolysis, making it a point of regulation for the entire pathway. The high energy cost of phosphorylating glucose makes hexokinase a key regulatory point, allowing the cell to control glycolytic flux.

**Why Each Wrong Option is Incorrect**
* **Option A:** Phosphofructokinase (PFK) is another regulatory enzyme in glycolysis, but it is not the key regulatory enzyme. PFK is responsible for the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, but it is not the initial step in glycolysis.
* **Option B:** Pyruvate kinase is the final enzyme in glycolysis, catalyzing the conversion of phosphoenolpyruvate (PEP) to pyruvate. While it is a regulatory enzyme, it is not the key regulatory enzyme of glycolysis.
* **Option D:** Glucose-6-phosphate dehydrogenase is an enzyme involved in the pentose phosphate pathway, not glycolysis.

**Clinical Pearl / High-Yield Fact**
The hexokinase enzyme is inhibited by glucose-6-phosphate, providing a negative feedback mechanism to regulate glycolytic flux. This is a key concept to remember when understanding the regulation of glycolysis in different cellular contexts.

**Correct Answer: A. Hexokinase**