**Question:** A 15-year-old type I diabetic faints after injecting himself with insulin. He is administered Glucagon and rapidly recovers consciousness. Glucagon induces activity of:

A. Glucose-6-Phosphatase
B. Glucagon Receptor
C. Insulin Receptor
D. Insulinase

**Core Concept:** Glucagon is a hormone produced by alpha cells of the pancreas and plays a vital role in regulating blood glucose levels. In diabetes mellitus, insulin and glucagon counteract each other to maintain glucose homeostasis. When a diabetic patient, like the 15-year-old type I diabetic in this scenario, takes excessive insulin, it leads to hypoglycemia (low blood glucose) and subsequent symptoms like fainting.

**Why the Correct Answer is Right:** Glucagon acts on glucagon receptor, which is a G-protein coupled receptor, to stimulate the liver to produce glucose through the process of gluconeogenesis, glycogenolysis, and glycogen synthesis. Glucagon promotes the breakdown of glycogen to glucose, inhibits glycolysis, and activates gluconeogenesis to increase blood glucose levels. In this scenario, administering glucagon to the hypoglycemic patient restores blood glucose levels and triggers a rapid recovery from fainting.

**Why Each Wrong Option is Incorrect:**

A. Glucose-6-Phosphatase is an enzyme involved in glycogenolysis, not glucagon's primary target. It is not the correct answer because glucagon's primary action is not on this enzyme.

B. Glucagon receptor is correct, but the reason for choosing option C (Insulin Receptor) is incorrect. Glucagon receptor's primary action is to increase blood glucose levels, not to decrease them as in the scenario.

C. Insulin receptor is involved in lowering blood glucose levels, which is the opposite of what is required in this scenario. Insulin lowers blood glucose levels, while glucagon raises them.

D. Insulinase is an incorrect term. The correct answer is glucagon receptor, which increases blood glucose levels, helping the patient recover from hypoglycemia.

**Clinical Pearl:** Hypoglycemia is a life-threatening emergency in diabetics, and administering glucagon is the first-line treatment for an unresponsive hypoglycemic patient, especially in type 1 diabetes mellitus. The correct answer (glucagon receptor) ensures an increase in blood glucose levels, counteracting the effects of insulin and stabilizing the patient's condition.