Correct Option: A. Insulin Detailed Explanation 1. Role of Insulin: Insulin is a hormone produced by the pancreas, specifically by the beta cells in the islets of Langerhans. Its primary function is to regulate blood glucose levels. When you eat, carbohydrates are broken down into glucose, which enters the bloodstream. In response to rising blood glucose levels, the pancreas releases insulin. 2. Mechanism of Action: - Facilitating Glucose Uptake: Insulin facilitates the uptake of glucose by cells, particularly muscle and fat cells. It does this by promoting the translocation of glucose transporters (GLUT4) to the cell membrane, allowing glucose to enter the cells. - Glycogenesis: Insulin stimulates the liver to convert excess glucose into glycogen for storage (glycogenesis). This process helps lower blood glucose levels. - Inhibition of Gluconeogenesis: Insulin also inhibits gluconeogenesis, which is the production of glucose from non-carbohydrate sources in the liver, further helping to reduce blood glucose levels. 3. Other Functions of Insulin: - Insulin promotes the synthesis of fatty acids and inhibits the breakdown of fat (lipolysis), contributing to energy storage. - It also plays a role in protein synthesis by promoting amino acid uptake into cells. Why Other Options Are Incorrect B. Cortisol: - Cortisol is a steroid hormone produced by the adrenal glands and is primarily involved in the stress response. It increases blood glucose levels by promoting gluconeogenesis and the breakdown of fats and proteins. While it does raise blood glucose, it is not the primary regulator; rather, it acts in opposition to insulin, especially during stress. C. Adrenaline (Epinephrine): - Adrenaline is also produced by the adrenal glands and is involved in the "fight or flight" response. It increases blood glucose levels by stimulating glycogenolysis (the breakdown of glycogen to glucose) and gluconeogenesis. However, like cortisol, it is not the primary hormone for regulating blood glucose levels under normal physiological conditions. D. Thyroxine: - Thyroxine (T4) is a hormone produced by the thyroid gland that regulates metabolism and energy expenditure. While it can influence glucose metabolism indirectly by increasing the basal metabolic rate, it does not directly regulate blood glucose levels like insulin does. Summary of Key Points

• Insulin is the primary hormone for regulating blood glucose levels.
• It facilitates glucose uptake by cells, promotes glycogen storage, and inhibits glucose production.
• Cortisol and adrenaline raise blood glucose but are not primary regulators; they respond to stress.
• Thyroxine influences metabolism but does not directly regulate blood glucose.

Revision Summary

• Insulin is produced by the pancreas and lowers blood glucose levels.
• It promotes glucose uptake, glycogen storage, and inhibits glucose production.
• Cortisol and adrenaline raise blood glucose but are not primary regulators.
• Thyroxine affects metabolism but does not directly regulate blood glucose.