## **Core Concept**
Insulin is a key hormone involved in glucose metabolism. It promotes the uptake of glucose into cells and stimulates glycolysis, the breakdown of glucose to produce energy. Insulin exerts its effects by activating various enzymes in the glycolytic pathway.

## **Why the Correct Answer is Right**
The correct answer, **Phosphofructokinase-1 (PFK-1)**, is a crucial enzyme in the glycolytic pathway. Insulin activates PFK-1 by promoting the dephosphorylation and activation of this enzyme. PFK-1 catalyzes the conversion of fructose-6-phosphate into fructose-1,6-bisphosphate, a key regulatory step in glycolysis. By activating PFK-1, insulin enhances the rate of glycolysis, facilitating the utilization of glucose by cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Hexokinase is an enzyme involved in the initial step of glycolysis, converting glucose into glucose-6-phosphate. While insulin does influence glucose uptake and utilization, its primary action related to glycolysis is not directly on hexokinase.
- **Option B:** Pyruvate kinase is another enzyme in the glycolytic pathway, catalyzing the conversion of phosphoenolpyruvate (PEP) to pyruvate. Insulin does activate pyruvate kinase, but the most direct and significant impact related to the provided options is on PFK-1.
- **Option D:** Glucose-6-phosphatase is actually involved in gluconeogenesis and glycogenolysis, catalyzing the conversion of glucose-6-phosphate to glucose, which is the opposite of glycolysis. Insulin inhibits gluconeogenesis and glycogenolysis, but its action on glucose-6-phosphatase is inhibitory, not stimulatory like the effect on PFK-1.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that insulin resistance, a condition where cells in the body do not respond effectively to insulin, leads to decreased glucose uptake in muscles and adipose tissue and increased glucose production in the liver. This condition is a hallmark of type 2 diabetes mellitus. Understanding the mechanisms of insulin action, including its effects on glycolytic enzymes like PFK-1, is crucial for managing and understanding the pathophysiology of diabetes.

## **Correct Answer:** C. Phosphofructokinase-1.