## **Core Concept**
Insulin is a key anabolic hormone that regulates glucose and lipid metabolism. It promotes the uptake of glucose into cells, glycogen synthesis, and inhibits gluconeogenesis and glycogenolysis. One of its critical effects is on enzymes involved in glucose and lipid metabolism.

## **Why the Correct Answer is Right**
The correct answer, **D. Phosphofructokinase-2 (PFK-2)**, is an enzyme that plays a crucial role in the regulation of glycolysis. PFK-2 is involved in the production of fructose-2,6-bisphosphate, a potent activator of phosphofructokinase-1 (PFK-1), a key enzyme in the glycolytic pathway. Insulin activates PFK-2, which leads to an increase in fructose-2,6-bisphosphate levels, thereby stimulating glycolysis and promoting glucose uptake in cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Protein kinase A (PKA) is actually inhibited by insulin. PKA is involved in the signaling pathway of glucagon and epinephrine, which raise blood glucose levels by promoting gluconeogenesis and glycogenolysis. Insulin counteracts these effects.
- **Option B:** Pyruvate kinase is indeed involved in glycolysis but insulin's direct and notable effect on increasing the activity of an enzyme related to glycolytic pathway regulation is more accurately described with PFK-2. Pyruvate kinase is activated by insulin but through a less direct mechanism compared to the activation of PFK-2.
- **Option C:** Glucose-6-phosphatase is an enzyme involved in gluconeogenesis and glycogenolysis, and its activity is actually decreased by insulin. This decrease helps in lowering blood glucose levels by reducing glucose production in the liver.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that insulin resistance, often seen in type 2 diabetes, leads to decreased activity of PFK-2 and thus decreased glycolysis, contributing to hyperglycemia. Understanding insulin's effects on metabolic pathways is crucial for managing diabetes and related metabolic disorders.

## **Correct Answer: D. Phosphofructokinase-2 (PFK-2).**