## Core Concept
The first committed step in glycolysis refers to the initial irreversible reaction that commits glucose to being broken down through the glycolytic pathway. This step is crucial as it ensures that once glucose enters the pathway, it will proceed to pyruvate, generating a small amount of ATP and NADH in the process.

## Why the Correct Answer is Right
The correct answer, **Phosphofructokinase-1 (PFK-1)**, catalyzes the conversion of fructose-6-phosphate into fructose-1,6-bisphosphate. This step is considered the first committed step of glycolysis because it is irreversible under physiological conditions and commits the glucose molecule to being metabolized through glycolysis. PFK-1 is a key regulatory enzyme in the glycolytic pathway, influenced by various factors including ATP, citrate, and AMP levels, making it a critical point of control.

## Why Each Wrong Option is Incorrect
- **Option A:** Hexokinase catalyzes the conversion of glucose to glucose-6-phosphate, which is the first step in glycolysis but not considered the first committed step because glucose-6-phosphate can still be used for glycogen synthesis or the pentose phosphate pathway.
- **Option B:** Glucose-6-phosphatase is involved in gluconeogenesis and glycogenolysis, catalyzing the conversion of glucose-6-phosphate to glucose, which is the opposite of what occurs in glycolysis.
- **Option D:** Pyruvate kinase catalyzes the final step of glycolysis, converting phosphoenolpyruvate (PEP) to pyruvate, generating one molecule of ATP per converted molecule. This is not the first committed step.

## Clinical Pearl / High-Yield Fact
A key clinical correlation is that **tarui's disease** is caused by a deficiency of **PFK-1** (phosphofructokinase-1), leading to glycogen storage disease type VII. This condition affects the muscles and is characterized by exercise-induced muscle cramps, fatigue, and myoglobinuria.

## Correct Answer: C. Phosphofructokinase-1 (PFK-1).