**Question:** During fasting, which of the following is released from the muscles?

A. Insulin
B. Glucagon
C. Growth hormone
D. Epinephrine

**Core Concept:** In response to fasting or low blood glucose levels, the pancreas secretes hormones to regulate blood glucose levels and mobilize energy sources from various organs, including muscles.

**Why the Correct Answer is Right:** When blood glucose levels drop during fasting, the alpha cells of the pancreas secrete glucagon. Glucagon acts on alpha cells in the liver to stimulate gluconeogenesis (the production of glucose from non-carbohydrate sources) and glycogenolysis (the breakdown of glycogen to glucose), increasing blood glucose levels.

**Why Each Wrong Option is Incorrect:**

A. Insulin is secreted by beta cells in response to high blood glucose levels, promoting glucose uptake into cells for energy or storage and lowering blood glucose levels.
B. Epinephrine, also known as adrenaline, is secreted by the adrenal medulla during stress or hypoglycemia. It increases blood glucose levels by stimulating gluconeogenesis and glycogenolysis in the liver and by inhibiting glucose uptake in peripheral tissues. So, epinephrine is not the correct answer.
C. Growth hormone is produced by the anterior pituitary gland and promotes growth, protein synthesis, and lipolysis (fat breakdown) in various tissues. It does not play a direct role in regulating blood glucose levels.

**Clinical Pearl:** Understanding the hormonal regulation of blood glucose levels is crucial for maintaining a stable glucose homeostasis and preventing hypoglycemia (low blood glucose) or hyperglycemia (high blood glucose).

**Correct Answer:** D. Epinephrine (adrenaline) is released during fasting or hypoglycemia to raise blood glucose levels.