## Core Concept
Glucagon is a peptide hormone produced by the pancreas that plays a crucial role in glucose metabolism. It acts through glucagon receptors, which are G protein-coupled receptors. The primary effect of glucagon is to increase blood glucose levels by stimulating glycogenolysis and gluconeogenesis.

## Why the Correct Answer is Right
The liver is a primary target organ for glucagon, where it promotes glycogen breakdown and glucose production. Glucagon receptors are also found in the adipose tissue, where they facilitate lipolysis, and in the kidneys, to a lesser extent, where they can influence glucose production. However, glucagon receptors are notably absent in the muscles (skeletal and cardiac), as glucagon does not directly stimulate glucose production or significant glycogen breakdown in these tissues. Instead, muscles utilize glucose and fatty acids through different signaling pathways.

## Why Each Wrong Option is Incorrect
- **Option A: Liver** - Glucagon receptors are indeed found in the liver, where glucagon promotes glucose production through glycogenolysis and gluconeogenesis.
- **Option B: Adipose tissue** - Glucagon receptors are present in adipose tissue, where they stimulate lipolysis, contributing to increased fatty acid and glucose levels in the blood.
- **Option C: Kidneys** - Glucagon can act on the kidneys to increase glucose production through gluconeogenesis, indicating the presence of glucagon receptors.

## Why Option D is Correct but Explained Here as Incorrect in Context
- **Option D: Skeletal muscle** - This is the correct answer because glucagon receptors are not significantly found in skeletal muscle. Glucagon does not directly stimulate glucose production in skeletal muscles; instead, muscles take up glucose from the blood, and their glycogen metabolism is regulated by other factors.

## Clinical Pearl / High-Yield Fact
A key clinical point to remember is that glucagon's action increases blood glucose levels, making it useful in the treatment of hypoglycemia, particularly when intravenous glucose cannot be administered.

## Correct Answer: D. Skeletal muscle