**Core Concept**
The rate-limiting step in glycolysis is a crucial regulatory point that controls the flow of glucose through the glycolytic pathway. This step is catalyzed by an enzyme that is allosterically regulated by various factors, including ATP, NADH, and citrate.

**Why the Correct Answer is Right**
The correct answer is **Hexokinase (HK)**. Hexokinase is the first committed step in glycolysis, converting glucose into glucose-6-phosphate (G6P). This step is irreversible under physiological conditions and is the primary regulatory point of glycolysis. Hexokinase is tightly regulated by allosteric effectors, such as ATP, which inhibits the enzyme by binding to a specific site, and fructose-2,6-bisphosphate, which activates the enzyme.

**Why Each Wrong Option is Incorrect**
* **Option A:** Phosphofructokinase (PFK) is a key regulatory enzyme in glycolysis, but it is not the rate-limiting step. PFK catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate and is allosterically regulated by ATP, ADP, and citrate.
* **Option B:** Pyruvate kinase (PK) is another regulatory enzyme in glycolysis, catalyzing the conversion of phosphoenolpyruvate (PEP) to pyruvate. While PK is important for regulating glycolysis, it is not the rate-limiting step.
* **Option D:** Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key enzyme in glycolysis, catalyzing the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. However, GAPDH is not the rate-limiting step in glycolysis.

**Clinical Pearl / High-Yield Fact**
The rate-limiting step in glycolysis, hexokinase, is also known as glucokinase in the liver. Glucokinase has a higher Km for glucose than hexokinase, allowing it to respond to changes in blood glucose levels.

**Correct Answer:** A. Hexokinase (HK)