## Core Concept
Insulin is a hormone that regulates blood glucose levels by facilitating glucose uptake into cells. When administered, insulin acts rapidly to decrease blood glucose levels. The effects of insulin on glucose metabolism occur through various mechanisms, including increased glucose uptake in muscles and adipose tissue, decreased glucose production in the liver, and inhibition of lipolysis.

## Why the Correct Answer is Right
The correct answer, **D. Decrease in blood glucose levels**, is expected to occur first because insulin's primary action is to lower blood glucose levels. This is achieved through increased glucose uptake by cells, particularly muscle and adipose tissue, and decreased hepatic glucose production. This action begins within minutes of insulin administration, making it the earliest effect.

## Why Each Wrong Option is Incorrect
- **Option A:** Increased glucose production in the liver is the opposite of what insulin does. Insulin inhibits glucose production in the liver, so this cannot be the first effect of insulin.
- **Option B:** Increased lipolysis is also counteracted by insulin. Insulin inhibits lipolysis, reducing the breakdown of fats into fatty acids and glycerol, which can be used for glucose production. Therefore, this is not an immediate effect of insulin.
- **Option C:** While insulin does have effects on protein and fat metabolism, such as promoting protein synthesis and inhibiting protein degradation, these effects, although important, are not the first or most immediate effects observed, which are primarily related to glucose metabolism.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the onset of action of insulin varies depending on its type. Rapid-acting insulin analogs start to work within 15 minutes after injection, while short-acting insulin starts to work within 30 minutes. This variability is crucial for managing blood glucose levels effectively, especially in diabetic patients.

## Correct Answer: D. Decrease in blood glucose levels